Patent Application
20090269253
1. Field of the Invention
The present invention relates to a member for suppressing the production of hydrogen sulfide (hereinafter, referred to as “H2S”) in exhaust gas from vehicle or the like, and an exhaust gas-purifying catalyst using the H2S production-suppressing member. The H2S production-suppressing member according to the present invention can suppress the production of H2S at the time of engine idling after a high-speed running. The H2S production-suppressing member of the present invention can be used by itself and can also be used as an exhaust gas-purifying catalyst such as a three-way catalyst.
2. Description of the Prior Art
As catalysts for purifying HC, CO and NOx, in vehicle exhaust gases, three-way catalysts have been widely used. Such three-way catalysts are formed by supporting platinum-group metals, such as Pt and Rh, on porous oxide supports, such as alumina, ceria, zirconia, and ceria-zirconia. Also, the three-way catalysts oxidize and purify HC and CO, and at the same time, reduce and purify NOx. Because these catalytic reactions efficiently proceed in an atmosphere in which oxidizing components and reducing components are mostly present in equivalent amounts, the combustion of fuel in vehicle engines provided with the three-way catalysts is controlled such that it occurs at around the theoretical air-fuel ratio (stoichiometric) (A/F=about 14.6±0.2).
However, the three-way catalysts have a problem in that, if an exhaust gas atmosphere is directed toward to reduction, sulfur oxide in exhaust gas will be reduced to H2S, which is then emitted into the air. For example, ceria, having a function of adsorbing and releasing oxygen, comprises components essential in the three-way catalyst. However, in a vehicle engine provided with a three-way catalyst including ceria, there is a problem in that H2S is produced when an exhaust gas atmosphere is rich (reducing atmosphere), which occurs, for example, in an acceleration mode.
The mechanism of H2S production using ceria will now be explained. SO2 in exhaust gas is oxidized to SOx by a metal catalyst. Ceria readily adsorbs SOx, because it is an oxide having a relatively high basicity. It is believed that the adsorbed SOx is slowly concentrated on the catalyst support, and is reduced to H2S in a reducing atmosphere. Even a trace amount of H2S is sensed by the human nose, giving an unpleasant feeling, and thus the emission needs to be suppressed. In addition, γ-alumina, which is widely used as a catalyst support, also readily adsorbs SOx.
Herein, the use of Ni or Cu oxide as the component of the three-way catalyst can be considered. The Ni or Cu oxide can suppress the production of H2S, because it converts SO2 into SO3 or SO4 in an oxidizing atmosphere and stores sulfur components as sulfides, for example, Ni2S3, in a reducing atmosphere.
Japanese Patent Application Publication No. H 08-015554, for example, discloses an exhaust gas-purifying catalyst formed by supporting a noble metal on a support, which comprises a composite oxide of nickel-barium, alumina and ceria. The support captures sulfur oxides as sulfates by alumina and ceria in a lean atmosphere, and captures H2S by the composite oxide of nickel-barium in a reducing atmosphere. Thus, it can suppress the production of H2S.
Furthermore, Japanese Patent Publication No. 2000-515419 or Japanese Patent No. 02598817 discloses the suppressing the production of H2S using, as a support, a mixture of a ceria with NiO, Fe2O3 and the like. Also, Japanese Patent Application Publication No. H 07-194978 discloses the suppressing the production of H2S using a support comprising Ni and Ca, supported on ceria.
However, Ni or Cu is limitedly used for vehicle exhaust gas-purifying catalysts because it is an environmental loading substance. There is another problem that the inherent purification properties thereof will be deteriorated when barium, for example, is added to a three-way catalyst.
In addition, Japanese Patent Application Publication No. H 02-020561 discloses bismuth-containing catalysts capable of oxidizing and removing H2S. However, because these catalysts oxidize H2S in an oxidizing atmosphere, they cannot prevent the emission of H2S in a stoichiometric or reducing atmosphere.
The present invention has been made to solve the above-described problems occurring in the prior art, and it is an object of the present invention to suppress the production of H2S without using environmental loading substances such as nickel.
To achieve the above object, in one aspect, the present invention provides a member for suppressing the production of H2S, comprising: a sulfur-adsorbing portion comprising an oxide including at least ceria, and disposed upstream side of an exhaust gas; and a sulfur-releasing portion having surface acidity higher than that of the sulfur-adsorbing portion and being disposed downstream side of the sulfur-adsorbing portion.
In another aspect, the present invention provides a exhaust gas-purifying catalyst comprising a hydrogen sulfide production-suppressing member and a noble metal supported thereon.
The above and other objects and features of the present invention will become apparent from the following description of preferred embodiment, given in conjunction with the accompanying drawings, in which:
Various embodiments of the present invention will now be described in detail with reference to the accompanying drawings.
A H2S production-suppressing member according to the present invention comprises a sulfur-adsorbing portion and a sulfur-releasing portion. The sulfur-adsorbing portion comprises an oxide including at least ceria. For example, the sulfur-adsorbing portion can comprises a mixture of ceria powder with other oxide powders such as alumina powder, and can also comprises either ceria alone or an composite oxide alone comprising ceria. Examples of the composite oxide comprising ceria may include ceria-zirconia, alumina-ceria-zirconia and so on.
As the ceria of the sulfur-adsorbing portion, it is preferable to use ceria having a specific surface area of less than 5 m2/g . In this case, the oxygen adsorption and release properties of ceria are maintained while the SOx adsorption properties thereof are decreased. Thus, the degree of rich can be reduced, and at the same time, SOx can be released before it is reduced to H2S, and the production of H2S can be suppressed. Similarly, when the sulfur-adsorbing portion contains alumina, it is preferable to use θ-alumina having a specific surface area smaller than that of γ-alumina.
The sulfur-releasing portion has acidity higher than that of the sulfur-adsorbing portion. The acidity of the sulfur-releasing portion can be increased using a method of applying an oxide having acidity higher than that of ceria in the sulfur-adsorbing portion, or a method of increasing the basicity of the sulfur-adsorbing portion. Examples of oxides having acidity higher than ceria may include silica, silica-alumina composite oxide, zirconia-containing alumina, titania, titania-zirconia composite oxide and so on, and one or more selected from these oxides can be used in the present invention. Among them, it is preferable to use titania, onto which SOx is difficult to be adsorbed, because titania has high acidity. Also, although alumina or zirconia has relatively low acidity, it can be advantageously used in the present invention because the acidity thereof will be increased when it is coated with titania.
To increase the basicity of the sulfur-adsorbing portion, at least one selected from among, for example, alkaline earth metals and rare earth elements is supported on the sulfur-adsorbing portion. As a result, the basicity of the sulfur-adsorbing portion is increased, leading to an increase in its ability to adsorb SOx. Thus, the emission of SOx, for example, at the time of engine idling after a high-speed running, can be suppressed, so that the production of H2S can be suppressed. The supporting amount of said at least one selected from alkaline earth metals and rare earth elements is preferably in the range of 0.01˜0.5 mol per liter of the H2S production-suppressing member. At less than 0.01 mol, the effect of the supported metal or element will not be expressed. On the other hand, if the metal or element is supported in an amount of more than 0.5 mol, the effect thereof will be saturated, and at the same time, when a noble metal is supported on the sulfur-adsorbing portion, the activity of the noble metal will be reduced.
The sulfur-adsorbing portion is disposed upstream side of an exhaust gas, and the sulfur-releasing portion is disposed downstream side of the sulfur-adsorbing portion. For example, a pellet-shaped sulfur-adsorbing portion can be filled in an exhaust pipe upstream side of an exhaust gas, and a pellet-shaped sulfur-releasing portion can be provided downstream side of the sulfur-adsorbing portion. Alternatively, a honeycomb-shaped sulfur-adsorbing portion having a coating layer comprising, for example, ceria, formed on a honeycomb substrate, may be disposed upstream side of the exhaust gas, and a honeycomb-shaped sulfur-releasing portion having a coating layer comprising, for example, titania, formed on a honeycomb substrate, may be disposed downstream side of the sulfur-adsorbing portion. A coating layer comprising the sulfur-adsorbing portion may be formed on one honeycomb substrate upstream side of the exhaust gas, and a coating layer comprising the sulfur-releasing portion may be formed on the honeycomb substrate downstream side of the sulfur-adsorbing portion.
For example, in the case of an H2S production-suppressing member, in which a coating layer comprising the sulfur-adsorbing portion is formed on one honeycomb substrate on the upstream side of the exhaust gas, and a coating layer comprising the sulfur-releasing portion is formed on the honeycomb substrate on the downstream side of the sulfur-adsorbing portion, the sulfur-adsorbing layer containing the sulfur-adsorbing portion can be formed in a range of ¼-⅔ of the total length of the H2S production-suppressing member. If the length of the sulfur-adsorbing layer is less than ¼ of the total length of the H2S production-suppressing member, the oxygen adsorption and release functions of ceria will be excessively decreased, making it difficult to relieve a rich atmosphere and suppress the production of H2S. If the area of the sulfur-adsorbing layer is more than ⅔ of the total area of H2S production-suppressing member, on the other hand, the adsorption range of SOx will be increased, and at the same time, released SOx will be adsorbed again, making it difficult to suppress the production of H2S.
The H2S production-suppressing member according to the present invention can be supported with a noble metals such as Pt, Rh, Pd, Ir, or Ru, and thus can be used as an exhaust gas-purifying catalyst for suppressing the production of H2S, and preferably a three-way catalyst. Also, the supporting of the noble metal on the H2S production-suppressing member improves the H2S-suppressing performance of the member. The noble metal is preferably supported on at least the sulfur-adsorbing portion. When the noble metal is supported on the sulfur-adsorbing portion, the oxygen adsorption and release functions of ceria can be improved to reduce fluctuations in the atmosphere of exhaust gas. Thus, it will be easy to maintain the exhaust gas atmosphere at an approximately stoichiometric ratio, and a high activity of the three-way catalyst will be expressed.
However, when only the sulfur-adsorbing portion is supported with a necessary amount of the noble metal, the supporting density of the metal will be increased, so that deterioration such as grain growth will tend to occur during the use of the catalyst. For this reason, it is preferable to support the noble metal uniformly on both the sulfur-adsorbing portion and the sulfur-releasing portion.
The supporting amount of the noble metal is preferably 0.05-10 wt %. If the supporting amount is less than 0.05 wt %, the catalyst will not be practical as an exhaust gas-purifying catalyst, and if the noble metal is supported in an amount of more than 10 wt %, the effect thereof will be saturated, and at the same time, the preparation coat of the catalyst will be increased.
In the prior ceria-containing three-way catalyst, ceria is present throughout an exhaust gas, and thus SOx is adsorbed almost uniformly through the exhaust gas. However, in the H2S production-suppressing member according to the present invention, the sulfur-adsorbing portion comprising basic ceria is disposed upstream side of the exhaust gas, and the sulfur-releasing portion having surface acidity higher than that of the sulfur-adsorbing portion is disposed downstream side of the sulfur-adsorbing portion. Thus, SOx in exhaust gas is adsorbed on the sulfur-adsorbing portion on the upstream side, but is difficult to be adsorbed on the sulfur-releasing portion. In other words, according to the H2S production-suppressing member of the present invention, the adsorption range of SOx is narrower than that in the prior art, and thus the production of H2S is decreased.
Also, in the H2S production-suppressing member of the present invention, SOx adsorbed on the sulfur-adsorbing portion is released at a high temperature zone, but the released SOx is difficult to adsorb on the sulfur-releasing portion. Thus, the released SOx is prevented from being adsorbed again to produce H2S therefrom.
Moreover, in the sulfur-adsorbing portion, the degree of a rich atmosphere is reduced due to the oxygen adsorption and release properties of ceria, and exhaust gas having reduced richness is brought into contact with the sulfur-releasing portion. Thus, H2S becomes more difficult to produce in the sulfur-releasing portion. In addition, the exhaust gas-purifying catalyst supported with the noble metal is used, the oxygen adsorption and release functions of ceria can be further increased, and thus the production of H2S can be further suppressed.
As a result, the H2S production-suppressing member and exhaust gas-purifying catalyst of the present invention can effectively suppress the production and emission of H2S through the synergistic action thereof.
Hereinafter, the present invention will be described in further detail with reference to Examples and Comparative Examples. Like reference numerals denote like element even in different drawings.
The cordierite honeycomb substrate 1 (1.1-L volume, 103-mm diameter, 130-mm length, 400 cpsi cell density and 100-μm wall thickness) was prepared. A range of ½ of the length of the honeycomb substrate 1 from one end of the honeycomb substrate (i.e., a range of half of the length of the substrate from the upstream side of an exhaust gas) was wash-coated with a slurry containing, as main components, 90 parts by weight of θ-alumina powder (100 m2/g specific surface area) and 100 parts by weight of ceria-zirconia solid solution powder (CeO2:ZrO2=1:1 molar ratio and 85 m2/g specific surface area). Then, the coated slurry was dried at 120° C., and calcined at 650° C. for 3 hours, thus forming the sulfur-adsorbing layer 2. The sulfur-adsorbing layer 2 was formed in an amount of 190 g per liter of the honeycomb substrate 1.
Then, the surface of the honeycomb substrate, on which the sulfur-adsorbing layer 2 has not been formed, was wash-coated with a slurry containing θ-alumina powder as a main component, and the coated slurry was dried at 120° C., followed by calcining at 650° C. for 3 hours, thus forming the sulfur-releasing layer 3. The sulfur-releasing layer 3 was formed in an amount of 90 g per liter of the honeycomb substrate 1.
The honeycomb substrate 1 having the sulfur-adsorbing layer 2 and the sulfur-releasing layer 3 was immersed in an aqueous rhodium nitrate solution so that it was adsorbed and supported with rhodium. Then, the substrate 1 was taken out of the solution and is dried at 120° C., followed by calcining at 500° C. for 1 hour, so that Rh was supported uniformly throughout the substrate 1. Also, the honeycomb substrate was impregnation in a given amount of a given concentration of dinitrodiamine platinum solution so that it was adsorbed and supported with platinum. Then, the substrate was dried at 120° C., followed by calcining at 500° C. for 1 hour, so that Pt was supported uniformly throughput the substrate 1. Pt and Rh were supported in amounts of 1.0 g and 0.2 g, respectively, per liter of the honeycomb substrate 1.
The same honeycomb substrate 1 as in Example 1 was used, and the same slurry containing θ-alumina powder (the same as in Example 1) and ceria-zirconia solid solution powder (the same as in Example 1) as main components, was wash-coated throughout the honeycomb substrate 1. Then, the coated slurry was dried at 120° C., followed by calcining at 650° C. for 3 hours, thus forming the first coating layer 20. The first coating layer 20 was formed in an amount of 190 g per liter of the honeycomb structure.
Then, the surface of the first coating layer 20 on the downstream side corresponding to half of the length of the honeycomb substrate 1 was wash-coated with a slurry containing, as main components, 90 parts by weight of θ-alumina (100 m2/g specific surface area) and 20 parts by weight of TiO2-coated ZrO2 powder (TiO2:ZrO2=30:70). Then, the coated slurry was dried at 120° C., followed by calcining 650° C. for 3 hours, thus forming the sulfur-releasing layer 3. The sulfur-releasing layer 3 was formed in an amount of 20 g per liter of the honeycomb substrate. Also, the honeycomb substrate was supported with Pt and Rh in the same manner as in Example 1.
This Example is the same as Example 1, except for the composition of the sulfur-releasing layer 3. Hereinafter, a method of preparing the three-way catalyst of Example 3 will be described.
The same honeycomb substrate as in Example 1 was used, and the same sulfur-adsorbing layer 2 as in Example 1 was formed over the range of ½ of the length of the honeycomb substrate from one end of the substrate.
Then, a slurry containing, as main components, 90 parts by weight of θ-alumina powder (specific surface area of 100 m2/g) and 20 parts by weight of TiO2-coated ZrO2 powder (TiO2:ZrO2=30:70), was wash-coated in a range of the half length of the honeycomb substrate from the downstream-side end. Then, the coated slurry was dried at 120° C., followed by calcining at 650° C. for 3 hours, thus forming the sulfur-releasing layer 3. The sulfur-releasing layer 3 was formed in an amount of 110 g per liter of the honeycomb substrate 1. In addition, the honeycomb substrate 1 was supported with Pt and Rh in the same manner as in Example 1.
The same honeycomb substrate 1 as in Example 1 was prepared. The range of the half length of the honeycomb substrate 1 from one end thereof was wash-coated with a slurry containing, as main components, 90 parts by weight of θ-alumina powder (the same as in Example 1), 100 parts by weight of ceria-zirconia solid solution powder (the same as in Example 1) and a given amount of barium sulfate powder. Then, the coated slurry was dried at 120° C., followed by calcining at 650° C. for 3 hours, thus forming the sulfur-adsorbing layer 2. The sulfur-adsorbing layer 2 was formed in an amount of 190 g per liter of the honeycomb substrate 1. Ba was supported in an amount of 0.1 mol per liter of the honeycomb substrate 1.
Then, the range of the half length of the honeycomb substrate 1 from the downstream-side end was wash-coated with a slurry containing, as main components, 90 parts by weight of θ-alumina powder (the same as in Example 1) and 100 parts by weight of ceria-zirconia solid solution powder (the same as in Example 1). Then, the coated slurry was dried at 120° C., followed by calcining 650° C. for 3 hours, thus forming the sulfur-releasing layer 3. The sulfur-releasing layer 3 was formed in an amount of 190 g per liter of the honeycomb substrate 1. In addition, Pt and Rh were supported in the same manner as in Example 1.
In a same manner as in Example 4, a three-way catalyst according to Example 5 comprises a cordierite honeycomb substrate 1 and a coating layer 30 formed throughout the honeycomb substrate 1. The coating layer 30 on the upstream side corresponding to the half length of the honeycomb substrate is supported with La. La is supported in an amount of 0.1 mol per liter of the honeycomb substrate. Thus, the surface acidity of the coating layer 30 is higher on the downstream side corresponding to the half length of the honeycomb substrate 1 than on the upstream side. Also, the sulfur adsorbing layer 2 is formed on the upstream side corresponding to the half length of the substrate, and the sulfur-releasing layer 3 is formed on the downstream side corresponding to the half length of the substrate. The three-way catalyst of this Example was prepared in the same manner as in Example 4, except that lanthanum oxide powder was used instead of barium sulfate powder.
In the same manner as in Example 4, a three-way catalyst according to Example 6 comprises a cordierite honeycomb substrate 1 and a coating layer 30 formed throughout the honeycomb substrate 1. The coating layer 30 on the upstream side corresponding to the half length of the substrate is supported with Ba and La. Each of Ba and La is supported in an amount of 0.1 mol per liter of the honeycomb substrate 1. Thus, the surface acidity of the coating layer 30 is higher on the downstream side corresponding to the half length of the honeycomb substrate 1 than on the upstream side. Also, the sulfur adsorbing layer 2 is formed on the upstream side corresponding to the half length of the substrate, and the sulfur-releasing layer 3 is formed on the downstream side corresponding to the half length of the substrate. The three-way catalyst of this Example was prepared in the same manner as in Example 4, except that lanthanum oxide powder was used in addition to barium sulfate powder.
Example 7 is carried out in the same manner as in Example 1, except that the specific surface area of the ceria-zirconia solid solution contained in the sulfur-adsorbing layer is 3 m2/g.
A three-way catalyst of this Comparative Example comprises a cordierite honeycomb substrate 1 and a coating layer 30 formed throughout the honeycomb substrate 1, and the composition thereof is uniform throughout thereof. Hereinafter, a method of preparing this three-way catalyst will be described.
The same honeycomb substrate 1 as in Example 1 was used, and a slurry containing, as main components, 90 parts by weight of γ-alumina powder (specific surface area of 180 m2/g) and 100 parts by weight of ceria-zirconia solid solution powder (the same as in Example 1), was wash-coated throughout the honeycomb substrate 1. Then, the coated slurry was dried at 120° C., followed by calcining at 650° C. for 3 hours, thus forming a coating layer 21. The coating layer 21 was formed in an amount of 190 g per liter of the honeycomb substrate 1. In addition, Pt and Rh were supported in the same manner as in Example 1.
Test and Analysis
Table 1 below summarizes the oxide structure of each of the catalysts.
Each of the three-way catalysts was mounted in the exhaust system of an engine bench, controlled at a stoichiometric ratio. Then, the engine was run in the LA#4 mode, and accelerated to 80 km/hr in a full acceleration mode in which an accelerator pedal was strongly stepped on. Then, the operation mode was converted to an idling state. Just after conversion to the idling mode, the amount of H2S emitted was measured, and the measurement results are shown in
As shown in
Also, H2S production-suppressing members, in which Pt and Rh were eliminated from the three-way catalysts of Examples and Comparative Example, were tested in the same manner as described above. AS a result, the relative values of Examples to Comparative Example were equal to the values of the three-way catalysts shown in
While the invention has been shown and described with respect to the preferred embodiments, it will be understood by those skilled in the art that various changes and modification may be made without departing from the spirit and scope of the invention as defined in the following claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-002714 | Jan 2006 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2007/050548 | 1/10/2007 | WO | 00 | 7/3/2008 |
