Patent Application
20110030354
The present invention relates to a particulate combustion catalyst, to a particulate filter, and to an exhaust gas cleaning apparatus. More particularly, the present invention relates to a particulate combustion catalyst which realizes removal (through oxidation) of particulate matter discharged from a diesel internal combustion engine; to a particulate filter coated with the particulate combustion catalyst; and to an exhaust gas cleaning apparatus including the particulate filter coated with the particulate combustion catalyst.
Exhaust gas discharged from diesel engines contains nitrogen oxides (NOx) and particulate matter, and release of such substances into the atmosphere without any treatment is a main cause of air pollution. Therefore, demand has arisen for strict regulation for such substances. There has been proposed, as effective means for removing particulate matter, a flow-through oxidation catalyst for combustion of soluble organic fractions (SOFs), and a diesel exhaust gas trapping system employing a diesel particulate filter for trapping soot. However, for regeneration of such a particulate filter, particulate matter trapped therein must be continuously removed through oxidation.
Hitherto, a variety of continuous regeneration systems have been proposed, and examples thereof include a system employing a catalyst including a carrier made of an inorganic oxide (e.g., zirconium oxide, vanadium oxide, or cerium oxide), and an expensive noble metal (e.g., Pt) supported on the carrier (see, for example, Patent Document 1, 2, or 3); and a continuous regeneration method involving NO2 (see, for example, Patent Document 4). This continuous regeneration method requires provision, upstream of a particulate filter, of an oxidation catalyst (e.g., Pt) for oxidizing NO into NO2, and thus involves high cost. In addition, reaction involving NO2 is affected by the ratio of NOx to C, and many restrictions are imposed on the employment of this method.
Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. H10-047035
Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. 2003-334443
Patent Document 3: Japanese Patent Application Laid-Open (kokai) No. 2004-058013
An object of the present invention is to provide a particulate combustion catalyst which exhibits excellent heat resistance, which realizes removal of soot through oxidation at low temperature without employment of an expensive noble metal, and which enables oxidation reaction to proceed with the aid of only oxygen and thus realizes removal of soot through oxidation at low temperature regardless of the NOx concentration of exhaust gas. Another object of the present invention is to provide a particulate filter coated with the particulate combustion catalyst. Yet another object of the present invention is to provide an exhaust gas cleaning apparatus comprising the particulate filter coated with the particulate combustion catalyst.
In order to achieve the aforementioned objects, the present inventors have conducted extensive studies, and as a result have found that the objects can be achieved by employing, as a carrier of a particulate combustion catalyst, a specific oxide or a composite oxide having a specific composition; supporting, on the carrier, an oxide of at least one metal selected from the group consisting of Ba, Ca, Mg, and Sr; and supporting, on the carrier, metallic Ag or Ag oxide, which serves as a catalyst component. The present invention has been accomplished on the basis of this finding.
Accordingly, the present invention provides a particulate combustion catalyst characterized by comprising a carrier formed of zirconium oxide; an oxide of at least one metal selected from the group consisting of Ba, Ca, Mg, and Sr, the metal oxide being supported on the carrier in an amount, as reduced to metal, of 0.5 to 30 mass % with respect to the carrier; and metallic Ag or Ag oxide, which serves as a catalyst component and is supported on the carrier.
The present invention also provides a particulate combustion catalyst characterized by comprising a carrier formed of a zirconium-cerium composite oxide; an oxide of at least one metal selected from the group consisting of Ba, Ca, Mg, and Sr, the metal oxide being supported on the carrier in an amount, as reduced to metal, of 0.5 to 30 mass % with respect to the carrier; and metallic Ag or Ag oxide, which serves as a catalyst component and is supported on the carrier.
The present invention also provides a particulate combustion catalyst characterized by comprising a carrier formed of a composite oxide containing zirconium, cerium, and at least one metal selected from among Nd, La, Fe, Y, Pr, Ba, Ca, Mg, Sn, and Sr; an oxide of at least one metal selected from the group consisting of Ba, Ca, Mg, and Sr, the metal oxide being supported on the carrier in an amount, as reduced to metal, of 0.5 to 30 mass % with respect to the carrier; and metallic Ag or Ag oxide, which serves as a catalyst component and is supported on the carrier.
The present invention also provides a particulate filter characterized by being coated with any of the aforementioned particulate combustion catalysts. The present invention also provides an exhaust gas cleaning apparatus characterized by comprising a particulate filter coated with any of the aforementioned particulate combustion catalysts.
The particulate combustion catalyst of the present invention exhibits excellent heat resistance. Employment of the particulate combustion catalyst of the present invention realizes removal of soot through oxidation at low temperature without use of an expensive noble metal. When the particulate combustion catalyst is employed, since oxidation reaction proceeds with the aid of only oxygen, soot can be removed through oxidation at low temperature regardless of the NOx concentration of exhaust gas. Even when a catalyst system including the particulate combustion catalyst is exposed to a high-temperature atmosphere for a long period of time, degradation of the system can be suppressed.
The particulate combustion catalyst of the present invention employs a carrier formed of zirconium oxide, a carrier formed of a zirconium-cerium composite oxide, or a carrier formed of a composite oxide containing zirconium, cerium, and at least one metal selected from among Nd, La, Fe, Y, Pr, Ba, Ca, Mg, Sn, and Sr. Among catalysts having the same composition (except for a carrier), a catalyst including a carrier formed of a zirconium-cerium composite oxide tends to be higher in performance than a catalyst including a carrier formed of zirconium oxide, and a catalyst including a carrier formed of a composite oxide containing zirconium, cerium, and at least one metal selected from among Nd, La, Fe, Y, Pr, Ba, Ca, Mg, Sn, and Sr tends to be higher in performance than a catalyst including a carrier formed of a zirconium-cerium composite oxide.
When the particulate combustion catalyst of the present invention employs, as a carrier, a zirconium-cerium composite oxide, the cerium oxide content of the composite oxide is preferably 5 to 50 mass %. When the cerium oxide content exceeds 50 mass %, the specific surface area of the carrier is reduced at a high temperature (e.g., 700° C. or higher), which may eventually cause thermal degradation of the catalyst. In addition, when the cerium oxide content exceeds 50 mass %, an active species may fail to sufficiently exert its performance. In contrast, when the cerium oxide content is less than 5 mass %, the carrier exhibits poor heat resistance, which may eventually cause thermal degradation of the catalyst.
When the particulate combustion catalyst of the present invention employs, as a carrier, a composite oxide containing zirconium, cerium, and at least one metal selected from among Nd, La, Fe, Y, Pr, Ba, Ca, Mg, Sn, and Sr, since the carrier contains an oxide of at least one metal selected from among Nd, La, Fe, Y, Pr, Ba, Ca, Mg, Sn, and Sr, the carrier exhibits improved thermal stability, and oxidation property at low temperature is improved. In order to attain such effects, the amount of an oxide of at least one metal selected from among Nd, La, Fe, Y, Pr, Ba, Ca, Mg, Sn, and Sr is preferably 1 mass % or more. However, when the amount of such a metal oxide exceeds 35 mass %, accordingly, the relative amounts of zirconium oxide and cerium oxide are reduced, and characteristics of the carrier containing the zirconium-cerium composite oxide tend to be deteriorated. Therefore, in the composite oxide contained in the carrier employed, preferably, the amount of an oxide of at least one metal selected from among Nd, La, Fe, Y, Pr, Ba, Ca, Mg, Sn, and Sr is 1 to 35 mass % (i.e., when two or more metal oxides are employed, the total amount of the oxides is 1 to 35 mass %), and the cerium oxide content is 5 to 50 mass % (zirconium oxide content: balance).
In the present invention, an oxide of at least one metal selected from the group consisting of Ba, Ca, Mg, and Sr must be supported on any of the aforementioned carriers. A conventionally known technique (e.g., the impregnation method or the sol-gel method) may be employed for supporting such a metal oxide on the carrier. In the present invention, when an oxide of at least one metal selected from the group consisting of Ba, Ca, Mg, and Sr is supported on the carrier, the resultant particulate combustion catalyst exhibits improved heat resistance. In order to attain such effects, the amount (as reduced to metal) of an oxide of at least one metal selected from among Ba, Ca, Mg, and Sr is preferably 0.5 mass % or more with respect to the carrier. However, when the amount (as reduced to metal) of such a metal oxide exceeds 30 mass % with respect to the carrier, accordingly, the relative amounts of zirconium oxide and cerium oxide are reduced, and characteristics of the carrier containing the zirconium-cerium composite oxide tend to be deteriorated. Therefore, preferably, the amount (as recued to metal) of an oxide of at least one metal selected from among Ba, Ca, Mg, and Sr is 0.5 to 30 mass % (i.e., when two or more metal oxides are employed, the total amount (as recued to metal) of the oxides is 0.5 to 30 mass %) with respect to the carrier (i.e., 0.5 to 30 parts by mass on the basis of 100 parts by mass of the carrier).
In the present invention, metallic Ag or Ag oxide must be supported, as a catalyst component, on the carrier. A conventionally known technique (e.g., the impregnation method or the sol-gel method) may be employed for supporting such a catalyst component on the carrier. Metallic Ag or Ag oxide, which is employed in the present invention, is less expensive than, for example, Pt or Pd. In addition, when metallic Ag or Ag oxide is employed in combination with a specific carrier used in the present invention, further excellent effects are obtained, as compared with the case where a Pt or Pd component is employed. In the present invention, preferably, the amount (as reduced to metal) of metallic Ag or Ag oxide supported on the carrier is 0.1 to 25 mass % on the basis of the total mass of the carrier and an oxide of at least one metal selected from the group consisting of Ba, Ca, Mg, and Sr. When the amount of the catalyst component is less than 0.1 mass %, the catalyst component may fail to sufficiently exhibit its catalytic effects, whereas when the amount of the catalyst component exceeds 25 mass %, a specific combination employed in the present invention may fail to sufficiently exhibit a synergistic effect. Meanwhile, when the amount of the catalyst component is large, sintering of metal is likely to occur, and the catalyst component is not expected to exhibit its catalytic effects.
In consideration that the particulate filter of the present invention is produced by causing the particulate combustion catalyst of the present invention to be held on a base, preferably, the surface of the carrier is provided with a binder component such as SiO2, TiO2, ZrO2, or Al2O3. When such a binder component is provided on the surface of the carrier, adhesion between the base and the carrier is enhanced, and the catalyst exhibits improved durability and heat resistance.
The particulate filter of the present invention may assume any known form of particulate filter, but preferably has a three-dimensional structure. Specific examples of filters having a three-dimensional structure include a wall-through filter, a flow-through honeycomb filter, a wire mesh filter, a ceramic fiber filter, a metallic porous filter, a particle-charged filter, and a foam filter. Examples of the material of the base include ceramic materials such as cordierite and SiC; Fe—Cr—Al alloys; and stainless steel alloys.
The exhaust gas cleaning apparatus of the present invention, which includes therein the aforementioned particulate filter of the present invention, will be readily appreciated by those skilled in the art.
Next will be described a method for producing the particulate filter of the present invention.
Any of the aforementioned types of carriers is mixed with a binder component (e.g., SiO2 or alumina sol) and water, and the resultant mixture is finely milled by means of a milling apparatus (e.g., a ball mill). A particulate filter (e.g., a wire mesh filter) is coated with the thus-obtained slurry. In general, the slurry-coated filter is fired at a temperature of about 500° C. to about 700° C. The thus-formed wash-coating layer is impregnated with, for example, a nitrate of at least one metal selected from the group consisting of Ba, Ca, Mg, and Sr, and then drying and firing are carried out. Subsequently, the resultant product is impregnated with, for example, silver nitrate serving as a catalyst component, and then drying and firing are carried out. Alternatively, the wash-coating layer may be impregnated with, for example, a nitrate of at least one metal selected from the group consisting of Ba, Ca, Mg, and Sr, together with, for example, silver nitrate, followed by drying and firing. The total catalyst coating amount is preferably 10 to 100 g/L (for a wall-flow particulate filter) or about 50 to about 150 g/L (for a wire mesh particulate filter). When the total catalyst coating amount is excessively small, sufficient performance fails to be attained, whereas when the total catalyst coating amount is excessively large, back pressure to exhaust gas increases.
The present invention will next be described in detail with reference to Examples and Comparative Examples. In each of the Examples and Comparative Examples, a parenthesized numerical value following each of the oxides constituting a composite oxide represents the amount (mass %) of the constitutive oxide.
Water (30 g) was added to powder of a composite oxide of CeO2(22)ZrO2(72)La2O2(2)Nd2O3(4) (20 g), and SiO2 sol (i.e., a binder component) (5 g, as reduced to SiO2) was added thereto, followed by mixing for two hours, to thereby prepare a slurry. By use of the slurry, a cordierite-made particulate filter (25.4 mm in diameter×60 mm in length) was coated with the composite oxide. The composite-oxide-coated filter was dried at 120° C. for three hours, and then fired in air at 500° C. for one hour. The composite-oxide-coated filter was found to have a composite oxide content of 40 g/L. The composite-oxide-coated filter was impregnated with an aqueous magnesium nitrate solution having a specific concentration and an aqueous silver nitrate solution having a specific concentration. The resultant product was dried at 120° C. for three hours, and then finally fired in air at 500° C. for one hour. The finally formed filter was found to have an Ag content of 5 g/L and an Mg content of 1 g/L. The Ag content as determined on the basis of the total mass of the aforementioned composite oxide and magnesium oxide was 12 mass %, and the Mg content as determined on the basis of the mass of the aforementioned composite oxide was 2.5 mass %.
The procedure of Example 1 was repeated, except that the aqueous magnesium nitrate solution was replaced with an aqueous calcium nitrate solution. The finally formed filter was found to have an Ag content of 5 g/L and a Ca content of 1 g/L. The Ag content as determined on the basis of the total mass of the aforementioned composite oxide and calcium oxide was 12.1 mass %, and the Ca content as determined on the basis of the mass of the aforementioned composite oxide was 2.5 mass %.
The procedure of Example 1 was repeated, except that the aqueous magnesium nitrate solution was replaced with an aqueous barium nitrate solution. The finally formed filter was found to have an Ag content of 5 g/L and a Ba content of 1 g/L. The Ag content as determined on the basis of the total mass of the aforementioned composite oxide and barium oxide was 12.2 mass %, and the Ba content as determined on the basis of the mass of the aforementioned composite oxide was 2.5 mass %.
The procedure of Example 1 was repeated, except that the aqueous magnesium nitrate solution was replaced with an aqueous strontium nitrate solution. The finally formed filter was found to have an Ag content of 5 g/L and an Sr content of 1 g/L. The Ag content as determined on the basis of the total mass of the aforementioned composite oxide and strontium oxide was 12.1 mass %, and the Sr content as determined on the basis of the mass of the aforementioned composite oxide was 2.5 mass %.
The procedure of Example 1 was repeated, except that the composite oxide of CeO2(22)ZrO2(72)La2O3(2)Nd2O3(4) was replaced with ZrO2, and the aqueous magnesium nitrate solution was replaced with an aqueous barium nitrate solution. The finally formed filter was found to have an Ag content of 5 g/L and a Ba content of 1 g/L. The Ag content as determined on the basis of the total mass of ZrO2 and barium oxide was 12.2 mass %, and the Ba content as determined on the basis of the mass of ZrO2 was 2.5 mass %.
The procedure of Example 1 was repeated, except that the composite oxide of CeO2(22)ZrO2(72)La2O3(2)Nd2O3(4) was replaced with a composite oxide of CeO2(30)ZrO2(70), and the aqueous magnesium nitrate solution was replaced with an aqueous barium nitrate solution. The finally formed filter was found to have an Ag content of 5 g/L and a Ba content of 1 g/L. The Ag content as determined on the basis of the total mass of the aforementioned composite oxide and barium oxide was 12.2 mass %, and the Ba content as determined on the basis of the mass of the aforementioned composite oxide was 2.5 mass %.
Water (700 g) was added to powder of a composite oxide of CeO2(22)ZrO2(72)La2O3(2)Nd2O3(4) (100 g), and the resultant mixture was milled by means of a ball mill so as to attain a mean particle size of 1 μm or less. Subsequently, ZrO2 sol (i.e., a binder component) (10 g, as reduced to ZrO2) was added to the mixture, followed by mixing for two hours, to thereby prepare a slurry. By use of the slurry, a cordierite-made particulate filter (25.4 mm in diameter×76.2 mm in length) was coated with the composite oxide. The composite-oxide-coated filter was dried at 120° C. for three hours, and then fired in air at 500° C. for one hour. The composite-oxide-coated filter was found to have a composite oxide content of 40 g/L. The composite-oxide-coated filter was impregnated with an aqueous barium nitrate solution having a specific concentration and an aqueous silver nitrate solution having a specific concentration. The resultant product was dried at 120° C. for three hours, and then finally fired in air at 500° C. for one hour. The finally formed filter was found to have an Ag content of 2 g/L and a Ba content of 2 g/L. The Ag content as determined on the basis of the total mass of the aforementioned composite oxide and barium oxide was 4.7 mass %, and the Ba content as determined on the basis of the mass of the aforementioned composite oxide was 5 mass %.
The procedure of Example 1 was repeated, except that the aqueous magnesium nitrate solution was not employed. The finally formed filter was found to have an Ag content of 5 g/L, and the Ag content as determined on the basis of the mass of the composite oxide was 12.5 mass %.
The procedure of Example 7 was repeated, except that the aqueous barium nitrate solution was not employed. The finally formed filter was found to have an Ag content of 2 g/L, and the Ag content as determined on the basis of the mass of the composite oxide was 5 mass %.
The Tig (combustion initiation temperature) of soot corresponding to each of the catalyst-coated particulate filters produced in Examples 1 to 6 and Comparative Example 1 was measured through the following method.
A specific amount of a dispersion prepared by dispersing carbon (Printex-V (toner carbon), product of Degussa) (20 mg) in ethyl alcohol was added dropwise to each of the catalyst-coated particulate filters produced in Examples 1 to 6 and Comparative Example 1 (25.4 mm in diameter×60 mm in length) from above the filter, followed by drying at 100° C. for 10 minutes. Thus, carbon (20 mg) was deposited on one catalyst-coated particulate filter. The carbon-deposited filter was fixed at a center portion of a quartz-made simulated exhaust gas reaction tube. While a circulation gas having the below-described composition was caused to flow through the quartz reaction tube at the below-described flow rate, the temperature of the reaction tube was elevated at the below-described temperature elevation rate by means of an electric furnace, and CO and CO2 concentrations were measured at the outlet of the reaction tube by means of an infrared analyzer. The temperature as measured at the inlet of the catalyst-containing reaction tube when CO2 concentration reached 400 ppm (i.e., electric furnace control temperature) was regarded as Tig.
Gas composition: O2: 10%, H2O: 10%, N2: balance
Flow rate: 25 L/min
Temperature elevation rate: 10 degrees (° C.)/min
Table 1 shows the thus-measured Tig corresponding to the respective catalyst-coated particulate filters produced in Examples 1 to 6 and Comparative Example 1, as well as the compositions of the catalysts.
For evaluation of heat resistance of a catalyst-coated particulate filter, the balance point temperature of the filter was measured by use of actual exhaust gas. Each of the catalyst-coated particulate filters produced in Example 7 and Comparative Example 2, or each of the catalyst-coated particulate filters produced in Example 7 and Comparative Example 2 and then subjected to thermal treatment at 700° C. or 800° C. for 20 hours was placed in a stainless steel holder, and the holder was fixed in a quartz reaction tube. While a portion of exhaust gas discharged from a diesel generator engine (engine displacement: 0.2 L) (rotation speed: 3,000 rpm) was distributed to the quartz reaction tube at a flow rate of 30.8 L/min, the quartz reaction tube was heated from outside by means an electric furnace. After the temperature had reached 300° C., the quartz reaction tube was heated in a stepwise manner at 20 degrees (° C.)/10 min. The difference in pressure between the inlet and the outlet of the reaction tube containing the particulate filter was measured, and the temperature at which the pressure difference is zero was determined. The thus-determined temperature was regarded as balance point temperature. Table 2 shows the balance point temperatures of the respective catalyst-coated particulate filters. As is clear from data shown in Table 2, the catalyst-coated particulate filter of Example 7 (i.e., the particulate filter of the present invention) exhibits excellent heat resistance (i.e., an increase in balance point temperature is suppressed even after thermal treatment at a high temperature), as compared with the case of the catalyst-coated particulate filter of Comparative Example 2.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2008-105156 | Apr 2008 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2008/069242 | 10/23/2008 | WO | 00 | 10/14/2010 |
