JP5085176B2 - Exhaust gas purification catalyst and exhaust gas purification device - Google Patents

Description

  The present invention relates to an exhaust gas purification catalyst and an exhaust gas purification device, and more particularly to an exhaust gas purification catalyst that efficiently purifies and reduces particulate matter (PM) contained in exhaust gas of a diesel engine.

  Conventionally, a diesel particulate removal device (DPF) is used to remove particulate matter contained in exhaust gas of a diesel engine. This particulate matter is mainly produced from fuel, and is composed of an organic component (SOF) that is a flammable component and soot that is a flame retardant component. The combustion temperature of such a particulate component requires very high temperatures of 200 to 550 ° C. for organic components and 550 to 700 ° C. for soot. The organic component is purified early by loading a noble metal catalyst on the DPF, like a catalyzed soot filter (CSF), but the purification performance against soot is low. For this reason, as described in Patent Documents 1 and 2, a technique has been proposed in which oxidation during regeneration of DPF is promoted by a catalyst having ceria.

  However, the technique of collecting PM by DPF causes pressure loss of exhaust gas and forcibly performs DPF, which causes fuel consumption loss, DPF dissolution by PM combustion heat, and catalyst deterioration. For this reason, the technique which reduces the burden on the exhaust system of a motor vehicle by burning PM including soot at lower temperature is desired.

  As a method to reduce the burden on the exhaust system of automobiles, the regeneration frequency is reduced, the soot combustion efficiency is improved, and PM is continuously burned only with the catalyst of the catalytic converter without using the DPF. It is possible to make it. Recently, as disclosed in Patent Documents 3 and 4, it has been proposed to use a noble metal or a composite oxide as a low temperature combustion catalyst for PM. However, the catalysts disclosed in these documents are effective in soot combustion, but the combustion temperature is as high as 450 to 600 ° C.

Conventionally, it is known that nitrogen dioxide (NO ₂ ) is highly active for PM combustion. Patent Document 5 proposes to arrange a NO ₂ production catalyst upstream of the DPF, and Patent Document 6 proposes to apply a NO ₂ production catalyst to the DPF. A method of burning and removing using NO ₂ in a temperature range where combustion does not occur has been proposed. However, in such a method, under the condition where NO _x is low, the effect of promoting PM combustion is small, and as the temperature increases, the concentration equilibrium between NO and NO ₂ is biased toward the NO side. _The PM combustion promotion effect by ₂ is extremely small.

In Patent Document 7, in order to further improve the PM combustion efficiency by NO _2, to combine the NO ₂ generation catalyst and PM combustion catalyst it has been proposed. NO This technique comprises a first catalyst for NO ₂ generation, the conversion has been NO ₂ and a second catalytic reaction with PM, by coating in the form of two-layer or mixed layer, which is produced in the first catalytic PM is efficiently removed by reacting ₂ and PM with a second catalyst. However, in this technique, since the second catalyst is coated while being coated or mixed with the first catalyst, the contact probability between PM and the second catalyst is actually very low. May be less effective.

  Patent Document 8 proposes a three-dimensional structure in which a transition metal catalyst layer containing a transition metal is applied as a lower layer, and a noble metal catalyst layer in which a noble metal is supported on an inorganic oxide is applied as an upper layer on the lower surface. Yes. However, in this technique, since PM is a solid-solid reaction, the upper layer and the PM can contact each other, but the lower layer catalyst and the PM cannot contact each other, so that the performance of the lower layer catalyst cannot be fully exhibited.

JP 2004-42021 A (summary) JP 2001-73748 A (summary) JP 2000-502598 A (Claim 1) JP-A-8-173770 (summary) JP-A-1-318715 (Summary) JP 2003-293730 A (summary) JP 2001-263051 A (summary) JP 2001-157845 A (summary)

  By the way, since the exhaust gas temperature discharged from the diesel engine is as low as 200 to 450 ° C., it is difficult to perform soot combustion continuously in the exhaust gas temperature range without performing a regeneration process or the like. Therefore, there is a strong demand for a technique for burning soot at a lower temperature range. Further, regarding a catalyst for exhaust gas purification of a diesel engine, further heat resistance is required when a future large displacement and high output are assumed. Diesel exhaust gas has a relaxed requirement for heat resistance of the catalyst than the gasoline exhaust gas atmosphere, but when it is required to have a performance that can purify high-temperature exhaust gas directly under the engine, conventional noble metal catalysts have the same effect as gasoline exhaust gas. Similarly, agglomeration of noble metals occurs, and the performance of the oxides used as the support is lowered due to structural destruction. Therefore, it cannot be said that the heat resistance of the catalyst is sufficient. On the other hand, the composite oxide itself does not have a problem of heat resistance because it forms a structure by high-temperature firing, but in any case, low-temperature combustion performance against soot is necessary. Therefore, an object of the present invention is to provide an exhaust gas purification catalyst and an exhaust gas purification device that can combust PM in a lower temperature range and have excellent heat resistance.

  Further, depending on the operating conditions, there are also conditions where the soot does not burn at all. The PM accumulated at this time causes clogging of the DPF, and the pressure loss due to the DPF increases as the amount of accumulated PM increases. Therefore, it is necessary to periodically remove the PM. For this reason, the DPF is regenerated by raising the temperature of the DPF to around 600 ° C. by external energy and removing it by combustion. At present, this regeneration process has caused many disadvantages such as deterioration of fuel consumption, emission, and system complexity. In order to reduce this disadvantage, it is necessary to lower the temperature during reproduction or shorten the reproduction time. Accordingly, an object of the present invention is to provide an exhaust gas purification catalyst and an exhaust gas purification device that burn PM at a lower temperature and have a higher PM combustion rate at a higher temperature.

The exhaust gas purifying catalyst of the present invention is an exhaust gas purifying catalyst for purifying particulate matter containing carbon as a main component contained in exhaust gas discharged from an internal combustion engine. The composite oxide includes any one of a perovskite type LaMn oxide, a rutile type CoTa oxide, a delafossite type PtCo oxide, and a spinel type ZnCo oxide .

Further, the exhaust gas purification apparatus of the present invention includes the exhaust gas purification catalyst and the exhaust gas inside the filter capable of collecting particulate matter mainly composed of carbon contained in the exhaust gas discharged from the internal combustion engine. The catalyst is characterized in that a NO ₂ production catalyst for converting NO to NO ₂ is supported at the same time.

  In the present invention, by supporting Ag having a high activity against PM on the complex oxide having oxygen releasing ability, the oxygen releasing ability of the complex oxide can be extracted at a lower temperature, so that the combustion of PM is performed at a lower temperature. Can be taken down. In addition, since Ag is supported on the composite oxide, Ag which is an active species can be disposed at the contact interface with PM, and the reactivity with PM is further improved. Furthermore, the composite oxide has high heat resistance, and the catalyst of the present invention is excellent in heat resistance because it can suppress aggregation and volatilization of Ag due to the interaction effect of supporting Ag on the composite oxide. It becomes. Thus, since the exhaust gas purification catalyst of the present invention can burn PM at a lower temperature range and is excellent in heat resistance, even in high heat conditions assuming further large displacement and high output engines in the future, It can be used not only under the floor but also directly under the engine regardless of the exhaust layout. In addition, since PM can be burned at a low temperature, fuel consumption loss and catalyst deterioration due to forced regeneration can be suppressed, the burden on the exhaust system of the vehicle can be reduced, and when PM collection and regeneration are not performed. Further, it becomes possible to perform continuous combustion of PM using a catalyst.

In addition, it is known that the complex oxide having oxygen releasing ability has high NO ₂ absorption ability. By coexisting with the NO ₂ production catalyst, the produced NO ₂ is adsorbed on the surface of the complex oxide having oxygen releasing ability. By this, since the surface NO ₂ concentration is kept high, the reaction between NO ₂ and PM by Ag is further promoted, and the PM combustion rate is improved. Further, by forming a two-layer coat of an Ag-supported composite oxide catalyst and a NO ₂ generation catalyst, and setting the average pore size in the NO ₂ generation catalyst layer to 1 μm or more, a composite oxide catalyst supporting Ag is obtained. It becomes easy for PM to contact, and a combustion rate improves further.

  The method for preparing the composite oxide is not limited, but a nitrate decomposition method, an organic acid complex polymerization method, or the like can be suitably used. Further, the method of supporting Ag on the composite oxide is not limited, but an impregnation method, a precipitation method, or the like can be suitably used.

  Next, a specific use example of the present invention will be described, for example, when a filter for collecting PM in exhaust gas is installed in an exhaust passage of a diesel engine. The filter has a three-dimensional network structure and is effective in any form, such as foam metal and foam ceramic with sufficient PM collection function, non-woven fabric with metal and ceramic fibers superimposed, and wall flow type filter. However, a wall flow type filter is preferable from the viewpoint of collection efficiency and contact between PM and catalyst. 10 and 11 schematically show a diesel particulate filter (hereinafter abbreviated as DPF). This DPF has a honeycomb structure and includes a number of exhaust gas passages 2 and 3 extending in parallel to each other. An exhaust gas inflow passage 2 in which the downstream end of the DPF is closed by the plug 4 and an exhaust gas outflow passage 3 in which the upstream end is closed by the plug 4 are alternately provided in the front, rear, left and right, and the exhaust gas inflow passage 2 and the exhaust gas are provided. It is separated from the outflow channel 3 through a thin partition wall 5. In FIG. 10, the hatched portion indicates the plug 4 at the downstream end. The DPF has a filter body made of a porous material such as silicon carbide or cordierite, and the exhaust gas flowing into the exhaust gas inflow passage 2 is surrounded by the surrounding partition walls 5 as shown by arrows in FIG. It flows out into the adjacent exhaust gas outflow passage 3 through the air. That is, as shown in FIG. 12, the partition wall 5 has fine pores 6 that connect the exhaust gas inflow passage 2 and the exhaust gas outflow passage 3, and the exhaust gas passes through the pores 6. A catalyst coat layer 7 is formed on the wall surface of the exhaust gas flow path (exhaust gas inflow path 2, exhaust gas outflow path 3 and pores) of the DPF filter body. For example, the coating layer 7 is formed by mixing the catalyst powder of the example with water and a binder to form a slurry, wash-coating the slurry on the filter body, and baking it.

Next, an embodiment of the exhaust gas purifying apparatus of the present invention will be described. FIG. 14 shows a cross-sectional structure of the DPF filter 10 of the embodiment. As shown in FIG. 14, the filter 10 is obtained by coating the above-mentioned Ag-supporting composite oxide layer 12 on the surface of the filter base 11 and coating the NO ₂ generation catalyst layer 13 thereon. The Ag-supported composite oxide layer 12 is composed of a catalyst having a high specific surface area alumina, silica, zirconia, magnesia, titania, ceria, etc. supported on a noble metal such as Pt, Pd, Rh. Since most of the nitrogen oxides in the exhaust gas are exhausted as NO, a composite oxide having an oxygen releasing capacity supporting Ag is applied to the surface of the filter base material 11, and the NO ₂ generation catalyst layer 13 is overlaid thereon. To apply NO ₂ to the lower Ag-supporting composite oxide layer 12.

However, when uniform application of NO ₂ generation catalyst layer 13 on the upper layer of Ag-supporting composite oxide layer 12, since the PM is unable to contact the Ag-supporting composite oxide layer 12, into NO ₂ synthesizing catalyst layer 13 A pore having a size of 1 μm or more is provided. Thereby, PM can be contacted with Ag carrying | support composite oxide layer 12, and PM can be burned efficiently. FIG. 15 shows the particle size distribution of PM in the exhaust gas under various conditions. As shown in this figure, since most of PM is distributed to 1 μm or less, it is desirable that the average size of the pores is 1 μm or more. There are the following methods for generating the voids. One is to make the average particle size of the NO ₂ production catalyst 10 μm or more. When the average particle diameter is 10 μm or more, the average particle diameter of voids generated between the particles is 1 μm or more. In addition, when coating the NO ₂ production catalyst, it is also effective to add a pore-forming agent mainly composed of carbon to the slurry, coat it, and then burn it away by high-temperature heat treatment to produce pores. As the pore-forming agent, starch, carbon black, resin or the like can be used.

Further, according to the study by the present inventors, it has been found that the higher the temperature (around 600 ° C.), the greater the effect of the two layers. That is, in the configuration of only the NO ₂ generation catalyst as in Patent Document 6, the NO ₂ concentration becomes extremely low at a temperature near 600 ° C. due to the equilibrium, and therefore the reaction between NO ₂ and PM hardly proceeds. However, by adopting a two-layer structure of the NO ₂ production catalyst and the PM combustion catalyst, the produced NO ₂ is adsorbed by the complex oxide having the oxygen releasing ability of the PM combustion catalyst before returning to NO, PM combustion tends to proceed with a large amount of NO _{2 in the} vicinity of the catalyst.

The applied amount of the catalyst is preferably 20 to 60 g per 1 L of the filter in the lower Ag-supporting composite oxide layer 12. If it is 20 g or less, the surface inside the pores of the filter cannot be sufficiently covered, and the contact property with PM becomes worse. On the other hand, when the coating amount exceeds 60 g, pressure loss due to pore clogging increases. Further, in the upper NO ₂ generation catalyst layer 13, 10 to 30 g per 1 L of the filter is good. When the coating amount is 10 g or less, the NO ₂ generation capability becomes insufficient, and when the coating amount exceeds 30 g, the thickness of the NO ₂ generation catalyst layer 13 becomes thick and it becomes difficult for PM to contact the PM combustion catalyst.

Hereinafter, the operation and effects of the present invention will be described more specifically with reference to examples.
[Sample preparation]
Catalyst samples of Examples 1 to 7 and Comparative Examples 1 to 19 were prepared with the following volumes.
<Example 1>
0.01 mol each of lanthanum nitrate and manganese nitrate, which are commercially available special grade reagents, and an appropriate amount of distilled water were weighed to prepare a mixed solution A. Next, 3.6 g of sodium carbonate and an appropriate amount of distilled water were weighed to prepare a mixed solution B. The mixed solution B was mixed while rotating at 60 ° C. and 300 rpm, and the mixed solution A was added dropwise thereto at 7 ml / min (reverse coprecipitation method). The precipitate was filtered and washed with distilled water until the pH became neutral, dried at 200 ° C. for 2 hours, and then dried and solidified at 350 ° C. for 3 hours. This was sized to 2 μm or less, and then fired at 800 ° C. for 10 hours was designated as Catalyst A. 9 g of catalyst A, 1.57 g of silver nitrate and appropriate amounts of distilled water were weighed to prepare a mixed solution C. A solution obtained by evaporating and drying the mixed solution C with an evaporator was dried at 200 ° C. for 2 hours and then calcined at 600 ° C. for 2 hours (impregnation method). This was sized to make the catalyst B 2 μm or less. Example 1 was obtained by pulverizing and mixing 9.5 mg of this catalyst B with 0.5 mg of PM, a mortar and a pestle to a particle size of 2 μm or less, and making a tight contact.

<Example 2>
0.01 mol of cobalt nitrate which is a commercially available special grade reagent and an appropriate amount of distilled water were weighed to prepare a mixed solution D. Next, 0.01 mol of tantalum oxide, 1.3 g of sodium carbonate, and appropriate amounts of distilled water were weighed to obtain a mixed solution E. A mixed solution D and a mixed solution E were prepared by the reverse coprecipitation method in the same manner as described above, and designated as catalyst C. An appropriate amount of 9.9 g of catalyst C, 0.16 g of silver nitrate, and distilled water was weighed out and prepared by impregnation as described above to obtain catalyst D. Example 2 was obtained by pulverizing and mixing 9.5 mg of the catalyst D with 0.5 mg of PM, a mortar and a pestle to a particle size of 2 μm or less, and forming a tight contact.

<Example 3>
0.01 mol of cobalt nitrate, which is a commercially available special grade reagent, 0.01 mol of dinitronimine platinum nitrate, and appropriate amounts of distilled water are weighed to obtain a mixed solution F. Next, 2.5 g of sodium carbonate and an appropriate amount of distilled water were weighed to obtain a mixed solution G. A precipitate was prepared from the mixed solution F and the mixed solution G by the reverse coprecipitation method in the same manner as described above, and used as catalyst E. An appropriate amount of 9.9 g of catalyst E, 0.16 g of silver nitrate and distilled water was weighed and prepared by impregnation in the same manner as described above to obtain catalyst F. Example 3 was obtained by grinding and mixing 9.5 mg of this catalyst F with 0.5 mg of PM, a mortar and a pestle to a particle size of 2 μm or less, and making a tight contact.

<Example 4>
0.02 mol of cobalt nitrate, which is a commercially available special grade reagent, 0.01 mol of zinc nitrate and appropriate amounts of distilled water were weighed to prepare a mixed solution H. Next, 3.8 g of sodium carbonate and an appropriate amount of distilled water were weighed to prepare a mixed solution I. The mixed solution H and the mixed solution I were prepared by the reverse coprecipitation method in the same manner as described above, and designated as catalyst G. An appropriate amount of 9.9 g of catalyst G, 0.16 g of silver nitrate and distilled water was weighed and prepared by the impregnation method in the same manner as described above to obtain catalyst H.
Example 4 was prepared by grinding and mixing 9.5 mg of this catalyst H with 0.5 mg of PM, a mortar and a pestle to a particle size of 2 μm or less, and making a tight contact.

<Example 6 >
A catalyst K was obtained by aging the catalyst B in the atmosphere at 800 ° C. × 6 Hr. Example 6 was obtained by grinding and mixing 9.5 mg of this catalyst K with 0.5 mg of PM, a mortar and a pestle to a particle size of 2 μm or less, and making a tight contact.

<Example 7 >
The catalyst B was aged at 850 ° C. × 6 Hr in the atmosphere as catalyst L. Example 7 was prepared by pulverizing and mixing 9.5 mg of this catalyst L with 0.5 mg of PM, a mortar and pestle to a particle size of 2 μm or less, and making a tight contact.

<Comparative Example 1>
The PM powder collected from the diesel generator was used as a sample for Comparative Example 1.

<Comparative example 2>
A commercially available special grade reagent, dinitrodiamine platinum nitric acid solution (1.51 g), Al ₂ O ₃ (9.92 g), and distilled water were weighed in an appropriate amount to prepare a mixed solution L. These were evaporated to dryness with an evaporator, and silver was supported on Al ₂ O ₃ . This was dried at 200 ° C. and then calcined at 600 ° C. for 2 hours. Comparative Example 2 was prepared by grinding and mixing 9.5 mg of this catalyst M with 0.5 mg of PM, a mortar and pestle to a particle size of 2 μm or less, and making a tight contact.

<Comparative Example 3>
Comparative Example 3 was prepared by grinding and mixing 9.5 mg of the catalyst A with 0.5 mg of PM, a mortar and a pestle to a particle size of 2 μm or less, and making a tight contact.

<Comparative example 4>
A commercially available special grade silver oxide (catalyst N) (9.5 mg) was pulverized and mixed to a particle size of 2 μm or less with 0.5 mg of PM, a mortar and a pestle to make a comparative contact.

<Comparative Example 5>
A commercially available special grade reagent, lanthanum nitrate, 0.01 mol, aluminum nitrate, 0.01 mol, and distilled water were weighed in an appropriate amount to obtain a mixed solution M. Next, 3.4 g of sodium carbonate and an appropriate amount of distilled water were weighed to prepare a mixed solution N. A mixed solution M and a mixed solution N were prepared by the reverse coprecipitation method in the same manner as described above, and designated as catalyst O. 9 g of catalyst O, 1.57 g of silver nitrate and appropriate amounts of distilled water were weighed out and prepared by impregnation in the same manner as described above to obtain catalyst P. Comparative Example 5 was prepared by pulverizing and mixing 9.5 mg of this catalyst P with 0.5 mg of PM, a mortar and pestle to a particle size of 2 μm or less, and making a tight contact.

<Comparative Example 6>
An appropriate amount of 0.02 mol of lanthanum nitrate, which is a commercially available special grade reagent, and distilled water were weighed to prepare a mixed solution O. Next, 2.1 g of sodium carbonate and an appropriate amount of distilled water were weighed to obtain a mixed solution P. The mixed solution O and the mixed solution P were prepared by the reverse coprecipitation method in the same manner as described above, and designated as catalyst Q. 9 g of catalyst Q, 1.57 g of silver nitrate and appropriate amounts of distilled water were weighed out and prepared by the impregnation method in the same manner as above to obtain catalyst R. Comparative Example 6 was prepared by grinding and mixing 9.5 mg of this catalyst R with 0.5 mg of PM, a mortar and a pestle to a particle size of 2 μm or less, and making a tight contact.

<Comparative Example 7>
A mixed solution Q was prepared by weighing 0.02 mol of manganese nitrate, which is a commercially available special grade reagent, and distilled water in an appropriate amount. Next, 2.1 g of sodium carbonate and an appropriate amount of distilled water were weighed to prepare a mixed solution R. The mixed solution Q and the mixed solution R were prepared by the reverse coprecipitation method in the same manner as described above and used as the catalyst S. 9 g of catalyst S, 1.57 g of silver nitrate and appropriate amounts of distilled water were weighed out and prepared by the impregnation method in the same manner as described above to obtain catalyst T. Comparative Example 7 was obtained by grinding and mixing 9.5 mg of this catalyst T with 0.5 mg of PM, a mortar and a pestle to a particle size of 2 μm or less, and making a tight contact.

<Comparative Example 8>
1 g of the catalyst Q and 1 g of the catalyst S were pulverized and mixed with a mortar pestle and calcined at 800 ° C. for 10 hours to obtain a catalyst U. 9 g of catalyst U, 1.57 g of silver nitrate and appropriate amounts of distilled water were weighed out and prepared by the impregnation method in the same manner as above to obtain catalyst V. Comparative Example 8 was obtained by grinding and mixing 9.5 mg of this catalyst V with 0.5 mg of PM, a mortar and a pestle to a particle size of 2 μm or less, and making a tight contact.

<Comparative Example 9>
9 g of the above catalyst A, 19.9 g of dinitrodiammine platinum nitric acid solution and appropriate amounts of distilled water were weighed out and prepared by the impregnation method in the same manner as above to obtain catalyst W. Comparative Example 9 was prepared by grinding and mixing 9.5 mg of this catalyst W with 0.5 mg of PM, a mortar and pestle to a particle size of 2 μm or less, and making a tight contact.

<Comparative Example 10>
An appropriate amount of 9 g of the catalyst A, 2.5 g of palladium nitrate and distilled water was weighed out and prepared by the impregnation method in the same manner as described above to obtain a catalyst X. Comparative Example 10 was prepared by grinding and mixing 9.5 mg of this catalyst X with 0.5 mg of PM, a mortar and pestle to a particle size of 2 μm or less, and making a tight contact.

<Comparative Example 11>
Appropriate amounts of 0.04 mol, 0.01 mol, 0.05 mol, and distilled water of commercially available special grade silver nitrate, lanthanum nitrate, and manganese nitrate were weighed to obtain a mixed solution S. The mixed solution S was evaporated to dryness while mixing at 250 ° C. and 300 rpm, then dried at 200 ° C. for 2 hours, calcined at 350 ° C. for 3 hours, and then sized so that the particle size was 2 μm or less. The catalyst Y was calcined at 800 ° C. for 10 hours. Comparative Example 11 was obtained by grinding and mixing 9.5 mg of this catalyst Y with 0.5 mg of PM, a mortar and pestle to a particle size of 2 μm or less, and making a tight contact.

<Comparative Example 12>
9 g of the catalyst A and 1 g of the catalyst N were weighed and physically mixed with a mortar pestle. This was calcined at 800 ° C. for 10 hours and subjected to a solid phase reaction to obtain catalyst Z. Comparative Example 12 was prepared by grinding and mixing 9.5 mg of this catalyst Z with 0.5 mg of PM, a mortar and pestle to a particle size of 2 μm or less, and making a tight contact.

<Comparative Example 13>
Comparative Example 13 was prepared by grinding and mixing 9.5 mg of the above catalyst C with 0.5 mg of PM, a mortar and a pestle to a particle size of 2 μm or less, and forming a tight contact.

<Comparative example 14>
Comparative Example 14 was prepared by grinding and mixing 9.5 mg of the above catalyst E with 0.5 mg of PM, a mortar and a pestle to a particle size of 2 μm or less, and forming a tight contact.

<Comparative Example 15>
Comparative Example 15 was prepared by grinding and mixing 9.5 mg of the above catalyst G with 0.5 mg of PM, a mortar and a pestle to a particle size of 2 μm or less, and forming a tight contact.

<Comparative Example 16>
Comparative Example 16 was prepared by grinding and mixing 9.5 mg of the above catalyst I with 0.5 mg of PM, a mortar and a pestle to a particle size of 2 μm or less, and forming a tight contact.

<Comparative Example 17>
A mixed solution T was prepared by weighing 0.02 mol of cerium nitrate, which is a commercially available special grade reagent, and distilled water in an appropriate amount. Next, 3.2 g of sodium carbonate and distilled water are weighed in an appropriate amount to obtain a mixed solution U. A mixed solution T and a solution U were prepared by the reverse coprecipitation method in the same manner as described above, and used as catalyst AA. An appropriate amount of 9.0 g of catalyst AA, 1.57 g of silver nitrate and distilled water was weighed out and prepared by impregnation in the same manner as described above to obtain catalyst AB. Comparative Example 17 was obtained by grinding and mixing 9.5 mg of this catalyst AB with 0.5 mg of PM, a mortar and pestle to a particle size of 2 μm or less, and forming a tight contact.

<Comparative Example 18>
A catalyst AC was obtained by aging the catalyst AB at 800 ° C. for 6 hours in the air. 9.5 mg of this catalyst AC was pulverized to a particle size of 2 μm or less with 0.5 mg of PM and a mixed mortar and pestle to make a tight contact, and Comparative Example 18 was used.

<Comparative Example 19>
The catalyst AB was aged at 850 ° C. × 6 Hr in the atmosphere as catalyst AD. Comparative Example 19 was prepared by grinding and mixing 9.5 mg of this catalyst AD with 0.5 mg of PM with a mortar and pestle to a particle size of 2 μm or less and forming a tight contact.

Table 1 shows the components of the samples of Examples 1 to 7 and Comparative Examples 1 to 19, types of carriers, types of metals supported on the carriers, and crystal structures of the carriers. In addition, a PM combustion test was performed on the above samples, and the heat generation characteristics (DTA) of PM were investigated. In the combustion test, an EXSTER 6000TG / DTA manufactured by Seiko Instruments Inc. is used, a 10 mg sample is loaded into the test apparatus, dry air is supplied at a flow rate at which the space velocity (SV) is 60000 h ⁻¹ , and the temperature of the dry air The temperature was raised at 20 ° C./min. 1 to 8 show the heat generation characteristics of each sample, and Table 1 also shows the temperature at which the amount of heat generated during PM combustion peaks.

FIG. 1 shows a sample made of only PM (Comparative Example 1), a sample made of 0.76 wt% Pt / Al ₂ O ₃ + PM (5 wt%) (Comparative Example 2), and a perovskite complex oxide LaMnO ₃ + PM (5 wt). %) (Comparative example 3) shows the combustion characteristics. The combustion peak temperature of the sample consisting only of PM is 668 ° C. On the other hand, in a noble metal material such as 0.76 wt% Pt / Al ₂ O ₃ + PM (5 wt%) (Comparative Example 2), the first peak is 247 ° C., the second peak is 561 ° C., and the peak is two. Divided into two. As for each combustion peak, it is known from the generated gas analysis that the first peak is derived from the organic component and the second peak is derived from soot. From this, it can be seen that the noble metal-based material is effective in burning organic components, but the soot cannot be burned at such a low temperature. Regarding these mechanisms, since it is an oxidation reaction on the surface of the active species as in the case of HC gas, it is presumed that organic components that are thermally unstable than soot are likely to be dissociatively adsorbed to the active species at an early stage. In the case of these noble metal catalysts such as Pt and Pd, the performance is hardly affected by the amount of noble metal.

On the other hand, transition metal based complex oxides such as LaMnO ₃ + PM (5 wt%) (Comparative Example 3) hardly cause separation of combustion peaks unlike noble metal based materials. In such a complex oxide, the organic component does not burn early but burns with soot. In the case of a composite oxide, unlike a noble metal material, soot can be burned at a relatively low temperature. Therefore, it can be said that the transition metal-based composite oxide is effective against soot combustion. These mechanisms are considered to depend on the oxygen releasing ability accompanying the valence change of the composite oxide, and therefore, there is no combustion of the organic component on the low temperature side where oxygen is not released, and combustion occurs simultaneously with soot. That is, it is possible to burn PM at a lower temperature as the oxygen releasing ability is higher.

  By the way, since the exhaust gas temperature discharged from the current internal combustion engine is as low as 200 ° C. to 450 ° C., the organic component can sufficiently burn within the exhaust gas temperature range by using a noble metal-based material. Combustion can be performed at a somewhat low temperature by the above catalyst material and the efficiency can be slightly increased, but it is difficult to perform continuous combustion in the exhaust gas temperature range without performing regeneration treatment or the like.

In this respect, the present invention succeeds in improving the PM combustion characteristics by improving the oxygen releasing ability of the composite oxide by supporting Ag on the composite oxide having the oxygen releasing ability. . FIG. 2 shows the combustion characteristics of 10 wt% Ag / LaMnO ₃ + PM (5 wt%) (Example 1). As can be seen from FIG. 2, in Example 1, the PM combustion characteristics are lower in temperature than in Comparative Example 3. As shown in Table 1, the PM combustion peak temperature is greatly lowered from 454 ° C. to 417 ° C. This is considered to be due to the fact that Ag is supported on the complex oxide having oxygen releasing ability.

As can be seen from the PM combustion peak value of Comparative Example 4 shown in Table 1, Ag ₂ O is very active against PM. Ag ₂ O is considered to cause an oxidation reaction by contacting with PM as a reducing agent. However, Ag ₂ O has a property of volatilizing and cannot be used alone from the viewpoint of heat resistance.

By supporting Ag on the composite oxide as in the present invention, Ag is considered to be present as Ag or Ag ₂ O. Ag reduced by the reaction acts effectively by being supported on the composite oxide. That is, while suppressing the volatilization of the reduced Ag, Ag is forced to take in oxygen by changing the valence of the composite oxide in order to activate again. By repeating these steps, it is presumed that the oxygen release ability of the catalyst can be increased, and as a result, the PM combustion temperature can be lowered. In other words, it can be said that the oxygen releasing ability of the composite oxide can be brought out at a lower temperature by supporting Ag having high activity with respect to PM in the composite oxide having oxygen releasing ability.

According to the mechanism of the Ag-supporting composite oxide, oxygen is absorbed and held in an oxygen-excess atmosphere ( lean state), and when the oxygen concentration is lowered ( rich state), it has the property of releasing oxygen. When excess oxygen is contained in the exhaust gas, the carbon-containing suspended particulates in the exhaust gas are collected and the oxygen concentration in the exhaust gas decreases, or the carbon-containing suspended particulates accumulate on the catalyst and the surrounding oxygen concentration When the value decreases, it is possible to release active oxygen and burn the carbon-containing suspended fine particles on the catalyst.

  Here, the performance in the case of carrying Ag for each of the composite oxide having no oxygen releasing ability (Comparative Example 5), the single oxide having oxygen releasing ability and the physical mixed oxide thereof (Comparative Examples 6 to 8) is shown in FIG. Compare with In Comparative Example 5, the PM combustion peak value is 521 ° C. and the performance is poor, and this performance is considered to be due to Ag alone. In Comparative Examples 6 and 7, the PM combustion peak values are 468 ° C. and 435 ° C., respectively, and the performance is inferior to that of Example 1. This is thought to be due to the low oxygen releasing ability of the single oxide as the carrier. In Comparative Example 8 in which the PM is physically mixed, the PM combustion peak value is 445 ° C., and the combustion temperature cannot be lowered so much.

  When considering soot combustion in the diesel exhaust gas temperature range, it is possible to obtain the expected performance with a physical combination of a complex oxide without oxygen releasing ability, a single oxide with low oxygen releasing ability and a single oxide. Can not. Therefore, it is considered important to use a complex oxide having oxygen releasing ability.

  Here, the definition of the oxide having oxygen releasing ability will be specifically described. First, in the temperature-programmed desorption test (TPD) of oxygen in a He atmosphere and the temperature-programmed reduction test (TPR) of oxygen with hydrogen, the desorption start temperature or the temperature at which desorption reaches a peak is defined as the peak area. Is the amount released. The ease of desorption and the amount of release are defined as the release ability. In particular, oxides such as composite oxides cause valence changes due to changes in the atmosphere, and oxygen release ability is defined as oxygen in the oxide being absorbed and released. It is defined as a complex oxide having oxygen releasing ability.

Figure 13 _shows the results _{La 2 O 3, MnO 2,} LaMnO 3, and the oxygen release capacity of LaAlO ₃ was measured by _O 2 -TPD. In FIG. 13, the horizontal axis represents temperature and the vertical axis represents the amount of oxygen desorption, which indicates the ease of thermal oxygen desorption in a He atmosphere. La ₂ O ₃ releases oxygen at around 360 ° C., but the peak area is small and the amount of released oxygen is small. On the other hand, MnO ₂ starts releasing oxygen from around 400 ° C., and the released amount increases as the temperature rises as it is. Further, in LaMnO ₃ having a perovskite structure, which is a composite oxide of these, the oxygen release start temperature is further lowered, starting from around 250 ° C. On the other hand, it can be seen that LaAlO ₃ does not have oxygen releasing ability in the entire temperature range.

As described above, each oxide and composite oxide may or may not have oxygen releasing ability, and each oxygen releasing ability is different. Here, MnO ₂ has greatly improved oxygen releasing ability from around 600 ° C., whereas La ₂ O ₃ does not have oxygen releasing ability at 600 ° C. or higher. From this, when the results of Comparative Examples 6 and 7 are considered, it is presumed that the oxygen releasing ability of 600 ° C. or higher does not contribute to the PM combustion characteristics. This is presumably because the ease of oxygen release at a low temperature, that is, the oxygen desorption energy is an important factor in PM combustion, rather than the amount of oxygen released from the catalyst. From the above, in a temperature-programmed desorption test (TPD) in a He atmosphere, a substance that starts deoxygenating at 600 ° C. or less can be defined as having an oxygen releasing ability.

Next, a case where a noble metal other than Ag is supported on a complex oxide having oxygen releasing ability (Patent Document 4) will be examined. FIG. 4 shows combustion characteristics when Pt and Pd are supported on LaMnO ₃ (Comparative Examples 9 and 10). 4 that the performance described in Patent Document 4 is inferior to that of Example 1.

  In PM activity, it can be said that Ag is specifically high in the performance of noble metal elements. Considering the interaction with the composite oxide, it is considered that Ag can extract the valence change of the composite oxide more from the viewpoint of the performance improvement effect from the comparative example 3. Is unknown.

Next, the case where the addition method of Ag to the composite oxide is changed will be described. Comparative Example 11 is a case where Ag is dissolved in the composite oxide (Patent Document 1), and Comparative Example 12 is a case where Ag is solid-phase reacted with the composite oxide (Patent Document 3). The result is shown in FIG. From FIG. 5, it can be seen that Comparative Examples 11 and 12 have poorer performance than Example 1. First, with respect to Comparative Example 11, even if the Ag amount is the same (10 wt%), active species are buried because Ag is dissolved in the crystal structure of LaMnO ₃ , and contact is made with PM as a reducing agent. As a result, the above-described change in Ag valence cannot be derived. For this reason, it is thought that performance is inferior compared with the case where Ag is carried like Example 1. Regarding Comparative Example 12, since there is no heat resistance, active species are volatilized, and it is considered that only the performance of LaMnO ₃ is obtained. Therefore, as in Example 1, it is necessary to lower the oxygen releasing ability by interacting with the composite oxide while exposing the active species on the outermost surface of LaMnO ₃ , and further suppress the pseudo collection and volatilization of Ag. Therefore, it is considered that the addition of Ag to the composite oxide must be supported.

  Next, in addition to the perovskite structure, the Ag supporting effect was confirmed in the complex oxide having oxygen releasing ability. Comparative Example 13 is a rutile complex oxide, Comparative Example 14 is a delafossite complex oxide, Comparative Example 15 is a spinel complex oxide, and Comparative Example 16 is a complex oxide in which simple elements are mixed. Examples in which 1 wt% of Ag is carried on these are Examples 2 to 5, and the results are shown in FIG. From FIG. 6 and Table 1, it can be seen that the PM combustion peak temperature is lowered by supporting Ag in any composite oxide. Therefore, it can be said that PM combustion characteristics can be improved by supporting Ag on a complex oxide composed of any crystal structure and any element having oxygen releasing ability.

Next, the results of aging treatment (in the atmosphere) from the viewpoint of heat resistance of the Ag-supported composite oxide will be described with reference to FIGS. In FIG. 7, Comparative Examples 17 and 19 show the results before the aging treatment (Patent Document 2) and after the aging treatment at 850 ° C. when Ag is supported on CeO ₂ . Since it is widely known that CeO ₂ is an oxide that is very excellent in oxygen releasing ability, it is considered that Ag loading on CeO ₂ having high oxygen releasing ability is also effective. In Comparative Example 17, the PM combustion peak temperature is 388 ° C. and good performance before aging. However, in Comparative Example 19 in which aging is applied to Comparative Example 17, the PM combustion peak temperature is greatly degraded to 445 ° C. This is presumed to be due to thermal structural destruction of the base oxide CeO ₂ and Ag aggregation and volatilization. In contrast, as shown in FIG. 8 in comparison with Example 1 and Example 7, it can be seen that the PM combustion characteristics before and after aging of 10 wt% Ag / LaMnO ₃ do not change. This is because the structure formation temperature of the composite oxide is 600 ° C. to 1000 ° C., so that structural destruction does not occur in the aging temperature range, and Ag aggregation and volatilization is unlikely to occur due to the interaction between the composite oxide and Ag. It is guessed.

FIG. 9 shows comparison of combustion characteristics (Examples 1, 6, 7 and Comparative Examples 17, 18, 19) before aging of Ag / CeO ₂ and Ag / LaMnO ₃ and after aging treatment at 800 ° C. and 850 ° C. . In FIG. 9, the horizontal axis represents the aging treatment temperature in the atmosphere, and the firing temperature when the sample was prepared was used as the treatment temperature before aging in each example. 9, depending aging conditions Ag / CeO ₂ it can be seen that not be burned sufficiently PM in a practical exhaust gas temperature region deteriorated performance. Therefore, the expected performance cannot be exhibited unless Ag is supported on the composite oxide having heat resistance.

  In this way, the present invention can withstand use under the engine floor as well as directly under the engine, regardless of the exhaust layout, even under high heat resistance conditions assuming further large displacement / high output ENG. It can be.

<Example 8>
(Adjustment of catalyst powder)
The powder of the catalyst B used in Example 1 was adjusted, and the required amount of the catalyst in which Ag was supported on the composite oxide having oxygen releasing ability was adjusted. The final firing temperature in this case was 700 ° C.

An aqueous solution containing a predetermined amount of palladium nitrate and platinum nitrate was prepared. This aqueous solution and γ-alumina were placed in an eggplant flask and impregnated with a rotary evaporator. The obtained product was dried at 200 ° C. for 1 hour and then calcined at 700 ° C. for 2 hours to prepare Pt / Al ₂ O ₃ as NO ₂ production catalyst powder.

(Supported on DPF)
The Ag-supported composite oxide powder, water, SiO ₂ sol, and alumina balls were placed in a container, and wet pulverization was performed overnight in a ball mill to obtain a catalyst slurry. After dipping DPF in this catalyst slurry, it was pulled up and excess slurry was removed by air blow. Next, the DPF was dried at 200 ° C. for 2 hours, and the weight of the DPF was measured. This operation was repeated until a predetermined amount of the catalyst was supported on the DPF, and then calcined at 700 ° C. for 2 hours. Thus, an Ag-supporting composite oxide layer was formed in the lower layer on the surface of the DPF.

The NO ₂ production catalyst powder, water, SiO ₂ sol, alumina balls, and starch having a particle size of 1 μm as a pore-forming agent were placed in a container, and wet pulverization was performed overnight in a ball mill to obtain a catalyst slurry. After dipping DPF in this catalyst slurry, it was pulled up and excess slurry was removed by air blow. Next, the DPF was dried at 200 ° C. for 2 hours, and the weight of the DPF was measured. This operation was repeated until a predetermined amount of the catalyst was supported on the DPF, and then calcined at 700 ° C. for 2 hours. Thus, a NO ₂ generation catalyst layer was formed on the upper surface of the DPF.

(Supporting honeycomb of oxidation catalyst)
The oxide catalyst powder, water, SiO ₂ sol, and alumina balls shown in Table 2 were placed in a container, and wet pulverization was performed overnight with a ball mill to obtain a catalyst slurry. The cordierite honeycomb was dipped in this catalyst slurry and then pulled up, and excess slurry was removed by air blow. Next, the cordierite honeycomb was dried at 200 ° C. for 2 hours, and the weight of the cordierite honeycomb was measured. This operation was repeated until a predetermined amount of the catalyst was supported on the cordierite honeycomb, followed by firing at 700 ° C. for 2 hours.

(Converter production)
FIG. 16 shows a converter 20 manufactured in this example. The converter 20 includes a cordierite honeycomb 21 carrying an oxidation catalyst as described above from the upstream side of the exhaust gas flow, an Ag-supported composite oxide layer as a lower layer, and a NO ₂ production catalyst as an upper layer as described above. A DPF 22 in which a layer is formed is provided. In this embodiment, the capacity of the cordierite honeycomb 21 is 1 L, and the capacity of the DPF 22 is 2 L.

<Comparative Example 20>
A converter 20 shown in FIG. 16 was manufactured under the same conditions as in Example 8 except that the NO ₂ generation catalyst layer was not formed on the DPF.

<Comparative Example 21>
A converter 20 shown in FIG. 16 was manufactured under the same conditions as in Example 8 except that the NO ₂ generation catalyst layer was directly formed on the DPF without forming the Ag-supporting complex oxide layer on the DPF.

<Comparative Example 22>
A converter 20 shown in FIG. 16 was produced under the same conditions as in Example 8 except that no pore forming agent was used when the NO ₂ production catalyst layer was formed on the DPF.

<Comparative Example 23>
A converter 20 shown in FIG. 16 was produced under the same conditions as in Example 8 except that the catalyst was not supported on the DPF.

(Evaluation method)
Using the converter 20 produced as described above, an engine bench test that can be evaluated close to practical conditions was performed. FIG. 17 shows a test layout, in which the converter 20 is disposed immediately below the 2.2 L diesel engine 30 and an operation test was performed.

(Low temperature continuous combustion performance)
The operation of the diesel engine 30 was continued for 8 hours under the condition that the exhaust gas temperature immediately before the cordierite honeycomb 21 was about 350 ° C. The amount of increase in the weight of the converter 20 after the end of operation was taken as the PM deposition amount. The result is shown in FIG. As shown in FIG. 18, in Example 8, the amount of accumulated PM was the smallest, so it can be seen that PM discharged during operation was best removed by combustion.

(Forced regeneration performance)
The operation of the diesel engine 30 was continued under the condition that the exhaust gas temperature just before the DPF 22 was about 150 ° C., and PM was deposited. Next, the diesel engine 30 was operated under the condition that the exhaust gas temperature immediately before the DPF 22 was about 600 ° C., and the converter 20 was forcibly regenerated. In forced regeneration, fuel was added by post-injection, the DPF 22 was raised to the target temperature by the combustion reaction heat in the cordierite honeycomb 21 carrying the oxidation catalyst, and stopped at predetermined time intervals to measure the weight. Then, the time until PM combustion was completed was measured, and the result is shown in FIG. As shown in FIG. 19, in forced regeneration at 600 ° C., it was confirmed that the time required for completion of regeneration was much shorter in Example 8 and the combustion speed during regeneration was the fastest.

  The exhaust gas purifying catalyst and the exhaust gas purifying apparatus of the present invention are capable of purifying PM contained in exhaust gas at a low temperature and have a fast PM combustion speed. Is also applicable.

It is a graph which shows the relationship between the supply air temperature in the Example of this invention, and the emitted-heat amount of PM. It is a graph which shows the other relationship between the supply air temperature in the Example of this invention, and the emitted-heat amount of PM. It is a graph which shows the other relationship between the supply air temperature in the Example of this invention, and the emitted-heat amount of PM. It is a graph which shows the other relationship between the supply air temperature in the Example of this invention, and the emitted-heat amount of PM. It is a graph which shows the other relationship between the supply air temperature in the Example of this invention, and the emitted-heat amount of PM. It is a graph which shows the other relationship between the supply air temperature in the Example of this invention, and the emitted-heat amount of PM. It is a graph which shows the other relationship between the supply air temperature in the Example of this invention, and the emitted-heat amount of PM. It is a graph which shows the other relationship between the supply air temperature in the Example of this invention, and the emitted-heat amount of PM. It is a graph which shows the relationship between the heating temperature (heat-resistant temperature) in the Example of this invention, and the temperature at the time of PM combustion peak. It is a front view which shows the catalytic converter in embodiment of this invention. It is a sectional side view which shows the catalytic converter in embodiment of this invention. It is a partially expanded side sectional view which shows the catalytic converter in embodiment of this invention. It is a graph for demonstrating the oxygen releasing ability in this invention. It is sectional drawing which shows a part of DPF in the Example of this invention. It is a graph which shows the particle size distribution of PM in exhaust gas. It is a side view which shows the converter produced in the Example. It is a side view which shows the test layout in an Example. It is a graph which shows the deposition amount of PM in the low temperature combustion in an Example. It is a graph which shows the reproduction time in forced reproduction in an example.

Claims (9)

An exhaust gas purifying catalyst for purifying particulate matter mainly composed of carbon contained in exhaust gas discharged from an internal combustion engine, which is a composite oxide that absorbs and releases oxygen by changing the valence of atoms . To support Ag ,
The composite oxide includes any one of a perovskite-type LaMn oxide, a rutile-type CoTa oxide, a delafossite-type PtCo oxide, and a spinel-type ZnCo oxide . An exhaust gas, characterized in that the exhaust gas purification catalyst according to claim 1 is applied to the inside of a filter capable of collecting particulate matter mainly composed of carbon contained in exhaust gas discharged from an internal combustion engine. Purification equipment. The exhaust gas purifying catalyst according to claim 1 and NO contained in the exhaust gas are converted into NO ₂ inside a filter capable of collecting particulate matter mainly composed of carbon contained in exhaust gas discharged from the internal combustion engine. An exhaust gas purifying apparatus characterized in that a NO ₂ production catalyst for conversion into NOx is simultaneously carried. Claim 3 wherein the NO ₂ synthesizing catalyst and the exhaust gas purifying catalyst, characterized in that it is applied in the exhaust gas purifying catalyst, the NO ₂ synthesizing catalyst in the upper structure to the lower layer on the inside of the base material of the filter The exhaust gas purification apparatus according to 1. 5. The exhaust gas purification according to claim 3, wherein the NO ₂ generation catalyst is formed by supporting one or more elements selected from Pt, Pd, and Rh on a high specific surface area support. apparatus. 6. The exhaust gas purifying apparatus according to claim 5 , wherein the high specific surface area carrier comprises one or more selected from alumina, silica, titania, ceria, zirconia, and magnesia. The exhaust gas purifying apparatus according to any one of claims 4 to 6 , wherein pores are present in the NO ₂ production catalyst layer. The exhaust gas purification apparatus according to claim 7 , wherein the pores have an average size of 1 µm or more. The exhaust gas purifying apparatus according to any one of claims 2 to 8 , wherein the filter is a wall flow type filter made of porous refractory ceramics.

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