CN1898019A - Pm oxidation catalyst and filter - Google Patents

Abstract

The catalyst suitable for diesel particulate filters (DPFs) that capture particulate matter (PM) in diesel engine exhaust gases is a perovskite-type composite oxide diesel exhaust particulate oxidation catalyst, which has a NO absorption region in the range of 200-450℃. This catalyst can induce low-temperature combustion of PM without the need for precious metals, thus it is inexpensive, and its constituent materials are non-volatile at exhaust gas temperatures, thus exhibiting good durability. This perovskite-type composite oxide is essentially free of Na and can be represented by the structural formula _RTO3 , where R includes one or more elements selected from La, Sr, Ba, Ca, and Li, and T includes one or more elements selected from Mn, Fe, Co, Cu, Zn, Ga, Zr, Mo, Mg, Al, and Si.

Description

PM Oxidation Catalysts and Filters

technical field

The present invention relates to a catalyst for burning particulate matter (PM) contained in exhaust gas of a diesel engine, and to a PM filter for controlling exhaust emissions from a diesel engine using the catalyst.

Background technique

Nitrogen oxides (NO _x ) and particulate matter (PM) are particular problems with respect to diesel exhaust. Among them, particulate matter includes fine particles mainly composed of carbon, and their most typical removal method is to place a diesel particulate filter (DPF) in an exhaust gas passage to capture the particulate matter. The trapped particulate matter is intermittently or continuously combusted to regenerate the particulate filter.

This filter regeneration process can be accomplished by burning particulate matter with electric heaters, burners, etc., or by carrying a catalyst in the particulate filter, whose catalytic effect reduces the ignition point of particulate matter, thereby Particulate matter is continuously burned at exhaust gas temperatures. The former method requires the addition of external energy and complicates the system, so the latter catalytic method is considered better.

Examples of such catalytic methods include the methods disclosed in Patent References 1 and 2 and Non-Patent References 1 and 2, in which platinum (Pt) is used as a catalyst metal. However, a method of solving the problem is an important issue due to an increase in cost caused by using a noble metal as a catalyst.

Patent Reference 3 details the use of a perovskite-type composite oxide in a DPF, and states that the ignition point of carbon black is lowered by its use.

Non-Patent Reference 3 proposes to use V ₂ O ₅ , MoO ₃ , PbO, Cs ₂ MoO ₄ , AgVO ₃ or their eutectic mixture as a melt moving type catalyst. These mixtures melt at exhaust gas temperatures, move across the surface of the honeycomb matrix, come into contact with particulate matter and cause it to oxidize and combust. Therefore, the lower the melting point of the mixture, the more mobile it is and the better it burns particulate matter at low temperatures, so these mixtures are thought to be better as catalysts. However, such low melting point materials are highly volatile, so they have a problem in that their durability is low. Therefore, these mixtures have not been put to practical use.

Non-Patent Reference 4 suggests the use of a potassium (K)-containing perovskite-type composite oxide. However, it is difficult for K to be completely contained in the structure of the perovskite-type composite oxide, so the K that cannot be incorporated into the structure exists in the form of oxide or hydroxide, and it is easy to flow into the moisture of the exhaust gas, causing durability problems again. question.

Patent Reference 1: JP Hei 11-253757A

Patent Reference 2: JP 2003-222014A

Patent Reference 3: JP Hei 06-29542B

Non-Patent Reference 1: Earozoru Kenkyu ["Aerosol Research," published by Japan Association of Aerosol Science and Technology] (2003), Vol. 18, No. 3, pp. 185-194

Non-Patent Reference 2: Jidosha Gijutsu Kai Gakujutsu Koenkai Maezurishu ["Proceedings of the Annual Congress of the Society of Automotive Engineers of Japan"] (2002), Vol. 22, No. 02, pp. 5-8

Non-Patent Reference 3: Kinzoku ["Metal" magazine] Vol. 74, (2004), No. 5, pp. 449-453

Non-Patent Reference 4: Nippon Seramikkusu Kyokai Gakujutsu Ronbunshi ("Journal of the Ceramic Society of Japan) (2003), Vol. 111, No. 129, pp. 852-856

Problems overcome by the present invention

One of the objects of the present invention is to provide a highly active and durable catalyst capable of burning particulate matter (PM) in exhaust gas of a diesel engine at low temperature. The catalyst is inexpensive because it does not contain precious metals, and its constituent materials do not volatilize at exhaust gas temperatures, so it has excellent durability. Another object is to provide a diesel particulate filter (DPF) using the catalyst to control exhaust emissions from diesel engines.

Contents of the invention

The above object is achieved by a diesel exhaust particulate matter oxidation catalyst using a perovskite type composite oxide having an NO absorption region in the whole or at least a part of the range of 200-450°C. The perovskite composite oxide can be represented by the structural formula RTO ₃ , wherein R is one or more elements selected from rare earth elements, alkali metal elements and alkaline earth metal elements except Na; T is selected from transition metal elements and Mg , one or more elements in Al and Si. In particular, a preferred composition is, wherein R includes one or more elements selected from La, Sr, Ba, Ca and Li, and T includes elements selected from Mn, Fe, Co, Cu, Zn, Ga, Zr, Mo , one or more elements of Mg, Al and Si. Note that Y is treated as a rare earth element.

In an exhaust gas atmosphere containing NO, the catalyst initiates combustion of particulate matter mainly composed of carbon in diesel engine exhaust at a temperature below 450°C. In addition, the present invention also provides a particulate matter filter for controlling exhaust emissions from diesel engines loaded with any of these catalysts.

The diesel particulate matter (PM) oxidation catalyst using a perovskite type composite oxide defined in the present invention can burn particulate matter collected in a diesel particulate filter (DPF) for controlling exhaust emissions from a diesel engine at a low temperature, thereby The amount of particulate matter released into the atmosphere is reduced and at the same time the load on various parts of the exhaust system is reduced since the temperature of the exhaust gas passing through the filter is lower than in the prior art. In addition, high activity catalysis can be achieved without the need to include noble metals, thus reducing the material cost of the particulate filter. Furthermore, the catalyst according to the present invention does not contain materials that volatilize at the exhaust gas temperature, so it also has excellent durability. Thus, the present invention improves the durability of the DPF system and greatly reduces the overall cost.

Brief description of the drawings

FIG. 1 shows an X-ray diffraction pattern of the perovskite-type composite oxide used in Example 1. As shown in FIG.

FIG. 2 is a graph showing the NO concentration and _CO concentration in the output gas as a function of the temperature of the heating process when the simulated diesel engine exhaust gas passes through the sample of perovskite-type composite oxide particles according to Example 1.

Fig. 3 is a graph showing the _CO concentration in the output gas changing with the temperature of the heating process when the simulated diesel engine exhaust gas passes through the honeycomb filter samples loaded with the catalysts obtained by the methods of Examples 1, 2 and Comparative Example 1.

Fig. 4 is a graph showing the _CO concentration in the output gas changing with the temperature of the heating process when the simulated diesel engine exhaust passes through the sample of the honeycomb filter loaded with the catalyst obtained by the method of Examples 2, 3 and Comparative Example 1.

Preferred Embodiments of the Invention

In a conventional catalytic type particulate filter, nitrogen monoxide (NO) contained in diesel engine exhaust gas is oxidized to nitrogen dioxide (NO ₂ ) on the surface of a metal catalyst such as Pt, and this NO ₂ is used for oxidation (combustion) mainly by Particulate matter consisting of carbon, which regenerates the filter.

In contrast, in the present invention, instead of a metal catalyst such as Pt, a perovskite-type composite oxide having an NO absorption region in a temperature range below 450°C (eg, 200-450°C) is used. According to the present inventor's research, it was found that this perovskite type composite oxide has a property of simultaneously causing a reaction of oxidizing (combusting) carbon particles into CO ₂ and releasing NO again at about 300-450°C. Particulate matter (PM) mainly composed of carbon contained in diesel engine exhaust generally has a combustion temperature of 500° C. or higher, so the perovskite type composite oxide used in the present invention has a catalytic effect of burning PM at low temperature.

This perovskite-type composite oxide can be represented by the general formula RTO ₃ . Wherein, R is one or more elements selected from rare earth elements (Y is also regarded as a rare earth element), alkali metal elements other than Na, and alkaline earth metal elements. However, it is preferable to include at least one or more of alkali metal elements and alkaline earth metal elements. Such a composition particularly remarkably exhibits the above-mentioned catalytic action. T is one or more elements selected from transition metal elements and Mg, Al and Si. There is no particular limitation on the rare earth elements constituting R, which may be Y, La, Ce, Nd, sm, Pr, etc., or preferably La. There are no special restrictions on the transition metal elements that make up T, which can be Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Zr, Mo, Ru, Rh, Pd, Ag, In, Sn, Pt, Au, etc., or preferably Ti, Mn, Fe, Co, Ni, Cu, Zn, Ga, Zr or Mo. Examples of elements constituting R other than rare earth elements include alkali metal elements and alkaline earth metal elements other than Na, and these elements may be contained in place of a part of the rare earth elements. Examples include Li, K, Ca, Sr, Ba, etc., but Li, Sr or Ba are preferred. From the viewpoint of obtaining a remarkable catalytic effect, it is preferable to make R contain at least one or more of Li, Ca, Sr or Ba, rather than make it contain only rare earth elements.

Patent Reference 3 discloses the use of a Na-based precipitant for the purpose of obtaining precipitation of composite oxides, but according to the research of the present inventors, it has been shown that in order to realize the catalytic effect of burning PM at low temperature, it is necessary to make the perovskite type composite oxidation The Na content in the product should be as small as possible, or specifically, the Na content should be 0.7% by mass or less, or more preferably zero. It is particularly difficult to remove the Na-based components incorporated in the raw materials. It was found that if a Na-based component is present as an impurity, as shown in the Examples below, the flash point tends to increase.

In a perovskite-type composite oxide having such a composition, it is realized to have an NO absorption region in a temperature range of 200-450°C, having a temperature range of about 300-450°C while causing oxidation (combustion) of carbon particles into CO ₂ The reaction and the characteristics of NO re-release. If cordierite or SiC or other honeycomb substrates are loaded with this perovskite composite oxide powder instead of traditional Pt catalysts, etc., then a high activity, high Durable Diesel Particulate Filter (DPF), which burns particulate matter (PM) in diesel exhaust at low temperatures.

The perovskite type composite oxide used in the present invention can be produced by a co-precipitation method, an organic complex method, an alkoxide method, or a method using, for example, an amorphous precursor. Various production methods are described below.

coprecipitation method

In the co-precipitation method, an aqueous solution of a raw material salt containing salts of the aforementioned elements in a stoichiometric ratio suitable for producing the perovskite-type composite oxide RTO ₃ is prepared, mixed with a neutralizing agent to cause coprecipitation, and then The resulting precipitate was dried and heat treated. The elemental salts used are not particularly limited, and any of their sulfates, nitrates, phosphates, chlorides or other inorganic salts, acetates, oxalates or other organic salts, etc. may be used. Among them, acetates and nitrates are particularly suitable. An aqueous solution of a raw material salt can be prepared by adding salts of the aforementioned elements to water so as to achieve a desired stoichiometric ratio and then stirring.

Then, the aqueous solution of the raw material salt is mixed with a neutralizing agent to cause coprecipitation. There is no particular limitation on the neutralizing agent used, and ammonia, potassium hydroxide and other inorganic bases, or triethylamine, pyridine or other organic bases can be used. In addition, the neutralizing agent is mixed until the pH of the slurry formed after addition of the neutralizing agent reaches 6-14. When mixed in this manner, highly crystalline co-precipitates of multiple elemental hydroxides can be obtained. The use of Na-containing bases is not preferred at this point because Na will be incorporated into the product.

If necessary, the coprecipitate thus obtained is rinsed with water and may be dried, for example by vacuum drying or forced air drying, followed by heat treatment at 600-1200°C, or preferably 800-1000°C, to obtain the desired perovskite type composite oxides. The atmosphere used in the heat treatment is not particularly limited as long as it is within the range of producing the perovskite type composite oxide, and an atmosphere of air, nitrogen, argon or hydrogen or one of them in combination with water vapor may be used, or preferably air or A nitrogen atmosphere or one of them in combination with water vapor.

Organic complex method

In the organic complex method, salts forming organic complexes of citric acid, malic acid, etc. and the aforementioned elemental salts can be added to water in a desired stoichiometric ratio and stirred to prepare an aqueous solution of the raw material salts.

An aqueous solution of a raw material salt is dried to form an organic complex of the aforementioned elements, which is then calcined and heat-treated to obtain a perovskite-type composite oxide.

The salts of these elements used may be the same as those used in the co-precipitation method. It is also possible to prepare an aqueous solution of a raw material salt by dissolving a mixture of raw material salts of various elements in a desired stoichiometric ratio, and then mixing it with an aqueous solution of an organic complex-forming salt. Note that the molar ratio of the organic complex-forming salt mixed in the mixture is preferably about 1.2 to 3 mol per 1 mol of the obtained perovskite-type composite oxide.

Thereafter, the raw material liquid is dried to obtain the aforementioned organic complex. There are no particular limitations on the drying conditions used as long as the temperature does not decompose the organic complex, for example, drying may be performed at room temperature to approximately 150°C, or preferably room temperature to 110°C for rapid moisture removal. The aforementioned organic complex is thus obtained.

The organic complex thus obtained is calcined and then heat-treated. Calcination can be performed by heating to a temperature of 250° C. or higher, for example, in vacuum or in an inert atmosphere. Then, heat treatment is performed at 600-1000° C. or preferably such as 600-950° C. to obtain the desired perovskite type composite oxide. At this time, the atmosphere used in the heat treatment is not particularly limited as long as it is within the range of producing the perovskite type composite oxide, and an atmosphere of air, nitrogen, argon or hydrogen or a combination of one of these gases and water vapor can be used , or preferably use an air or nitrogen atmosphere or a combination of either of them with water vapor.

Alkoxide method

In the alkoxide method, an alkoxide solution of a raw material containing alkoxides of the aforementioned elements in a stoichiometric ratio is prepared, and the raw material solution reacts with water to cause hydrolysis to obtain a precipitate. The precipitate thus obtained may be dried and then heat-treated to obtain the desired perovskite-type composite oxide.

Alcoholates of the elements used are not particularly limited as long as the elements are mixed uniformly, and any alcoholate formed of, for example, methoxy, ethoxy, propoxy, isopropoxy, butoxy and other alkoxy groups can be used. The alkoxide solution of the raw materials can be prepared by dissolving these alkoxides in an organic solvent so as to achieve the desired stoichiometric ratio, followed by stirring and mixing. The organic solvent that can be used is not particularly limited as long as it can dissolve the alkoxide of the element, so for example, benzene, toluene, xylene and the like can be used.

Then, water was added to the raw material solution to cause hydrolysis to obtain a precipitate. The obtained precipitate is rinsed with water if necessary, and may be dried by, for example, vacuum drying or forced-air drying, and then heat-treated at 500-1000° C. or preferably 500-850° C. to obtain the desired perovskite type composite oxide. The atmosphere used in the heat treatment is not particularly limited as long as it is within the range of producing the perovskite type composite oxide, an atmosphere of air, nitrogen, argon or hydrogen or a combination of one of these gases and water vapor may be used, or preferably Use air or nitrogen atmosphere or a combination of either of them and water vapor.

Methods Using Amorphous Precursors

In methods using amorphous precursors, as the present inventors have reported in Japanese Patent Application No. 2004-61882 (and corresponding US Patent Application No. 10/803,963 and European Patent Application No. 04007386.8) and Japanese Patent Application No. 2004 - Disclosed in 61901 (and corresponding U.S. Patent Application No. 10/809,709 and European Patent Application No. 04007387.6), containing the aforementioned elements in a stoichiometric ratio suitable for producing a perovskite-type composite oxide having the RTO ₃ structure A precursor material composed of a powdery amorphous material, which can be heat-treated at a low temperature to obtain a perovskite type composite oxide.

Such an amorphous precursor can be obtained by preparing an aqueous solution of a raw material salt containing the aforementioned elemental salt in a stoichiometric ratio suitable for producing a perovskite-type composite oxide having an _RTO3 structure, and combining the aqueous solution with a A carbonate of ammonium ion or an alkaline carbonate precipitant is reacted at a reaction temperature of 60° C. or lower and a pH of 6 or higher to form a precipitated product, and the filtrate is dried.

More specifically, an aqueous solution is prepared by first dissolving R's nitrate, sulfate, chloride or other aqueous inorganic salt and T's nitrate, sulfate, chloride or other aqueous inorganic salt in water, wherein R element and T The molar ratio of the elements is 1:1. The molar ratio of the R element to the T element is ideally 1:1, but a perovskite-type composite oxide may be formed even if it is not 1:1. Therefore, even if the molar ratio of the R element to the T element deviates slightly from 1:1, it is sufficient as long as the value is a value that allows the formation of the perovskite type composite oxide. Note that the R element may include two or more components, and the T element may also include two or more components. In this case, the different components should be dissolved such that the molar ratio of the total number of moles of elements making up R to the total number of moles of elements making up T is approximately 1:1.

The ion concentration of R and T in the solution in which the precipitate will form is such that, the upper limit is determined by the solubility of the salt used, and such that the crystalline compound of R or T does not precipitate out, but generally the total ion concentration of R and T is preferably around 0.01-0.60 mol/L range.

In order to obtain an amorphous precipitate from the solution, it is preferred to use a precipitating agent which is a carbonate or alkaline carbonate containing ammonium ions. Examples of such precipitating agents include ammonium carbonate, ammonium bicarbonate, etc., but Aqueous ammonia or another base may be added if desired. Furthermore, after forming a precipitate using ammonia water or the like, carbon dioxide may also be blown in to obtain an amorphous substance suitable as an amorphous precursor of the perovskite type composite oxide used in the present invention. When the amorphous precipitate will be obtained, it is best to control the pH value of the solution in the range of 6-11. If the pH value is lower than 6, it is not suitable, because the rare earth elements that make up R may not form precipitates. Conversely, if the pH is higher than 11, in the case of the precipitating agent alone, hydroxide or other crystalline precipitates may form without sufficient conversion of the formed precipitates to the amorphous form. In addition, the reaction temperature may be 60°C or lower. If the reaction is started above 60°C, particles of crystalline compounds of R or T may be formed, which can interfere with the conversion to the amorphous precursor and are therefore not preferred. The use of sodium-containing precipitants has been found to increase the flash point. The reason is considered to be as follows. If sodium is mixed into the precursor, it will remain at a concentration of about several hundred ppm no matter how it is cleaned, and this will have a detrimental effect on characteristics such as ignition point.

The obtained amorphous precursor is optionally rinsed with water, and may be dried by, for example, vacuum drying or forced-air drying, followed by heat treatment at 500-1000°C, preferably 500-800°C, to obtain the desired perovskite-type composite oxide thing. The atmosphere used in the heat treatment is not particularly limited as long as it is within the range of producing the perovskite type composite oxide, an atmosphere of air, nitrogen, argon or hydrogen or a combination of one of these and water vapor may be used, or preferably air or Nitrogen or a combination of one of them and water vapour.

Example

Example 1

Lanthanum nitrate, strontium nitrate and manganese nitrate were mixed so that the molar ratio of the elements lanthanum, strontium and manganese was 0.8:0.2:1.0. The mixture was added to water so that the total molar concentration of the elements lanthanum, strontium and manganese in the solution was 0.2 mol/L. While stirring the raw material solution, the temperature of the solution was adjusted to 25° C., and when the temperature reached 25° C., a mixed solution of ammonium carbonate and ammonia water was added as a precipitating agent, and the pH value was adjusted to 9 at the same time. Then, stirring was continued for six hours while maintaining the reaction temperature at 25° C., so that the formation of precipitates proceeded sufficiently. The precipitate thus obtained can be recovered by filtration, washed with water and dried at 110°C. The resulting powder is called a precursor powder.

Next, the precursor powder was heat-treated at 600° C. in air and calcined. Figure 1 shows the x-ray diffraction pattern of the calcined product thus obtained. By comparing the X-ray diffraction pattern of FIG. 1 with the JCPDS chart, it was determined that this calcined product was a substance having a crystal phase of (La _0.8 Sr _0.2 )MnO ₃ perovskite-type composite oxide.

Example 2

Except using lanthanum nitrate, strontium nitrate and ferric nitrate as raw materials and mixing the element lanthanum, the molar ratio of strontium and iron is 0.8:0.2:1.0, other all repeat embodiment 1.

As a result of X-ray diffraction analysis of the crystal structure, it was confirmed that the calcined product had a (La _0.8 Sr _0.2 )FeO ₃ perovskite-type composite oxide crystal phase. In addition, composition analysis by atomic absorption analysis showed that the Na value was less than 1 ppm (less than the measurement limit).

Example 3

A part of the perovskite-type composite oxide obtained in Example 2 was sampled and heat-treated at 800° C. for 24 hours. Heat treatment in air.

Comparative example 1

Commercial SiO ₂ (Wakogel C-100 manufactured by Wako Pure Chemical Industries, Ltd.) was impregnated with Pt with an aqueous solution of [Pt(NH ₃ ) ₄ ](OH) ₂ and dried at 120° C. for 12 hours by forced air. The impregnated product thus obtained was reduced at 400°C for 4 hours in 4% H ₂ (the balance being N ₂ ) and then oxidized at 500°C for 2 hours in air to obtain Pt-containing SiO ₂ . At this time, the mass percent content of Pt in SiO ₂ is 1%.

Comparative example 2

Except using sodium hydroxide as precipitation agent, other all repeat embodiment 2. The composition of the obtained perovskite-type composite oxide particle powder was analyzed by atomic absorption analysis, and the result showed that the mass percent content of Na was 0.77%.

Evaluation of PM Combustion Temperature Based on Particulate Samples

The combustion temperature of PM was evaluated according to the method described in Kankyo Hozen Kenkyu Seikashu ("Environmental Protection Research Results", National Institute of Industrial Health of Japan) (1999), 1, pp.37-1-37-13 below.

Each of the powders obtained in Example 1 and Comparative Example 1 was pressed using a die press of 500 kg/cm ² , and then pulverized to prepare a particulate sample with a particle diameter of 0.25-0.50 mm. As a simulated PM, commercial carbon black was added to these particulate samples to form a content of 1% by mass, and they were mixed by shaking in a glass bottle. The state of contact between the carbon and the catalyst sample obtained by this mixing method was a "loose contact" state, which approximated the state in which PM was actually trapped on the filter.

The aforementioned particulate samples mixed with carbon black were loaded into ventilated fixed beds, which were in contact with a constant flow of simulated diesel exhaust as shown in Table 1, so that the concentrations of CO _and NO in the gas passing through the vented fixed beds could be continuously measured. Then, once the flow of simulated diesel engine exhaust gas was started, the temperature was raised from room temperature to 800 °C at a heating rate of 10 °C/min while monitoring the _CO2 and NO concentrations in the gas passing through the ventilated fixed bed.

CO _{concentration} and NO concentration were detected using a Nicolet Nexus 470 FT-IR.

Figure 2 shows an example of changes in _CO2 concentration and NO concentration in Example 1. As can be seen from Figure 2, the downstream NO concentration drops rapidly at 200-250°C and shows values below 200 ppm throughout the range of approximately 250-350°C. This means that NO sucked into the air stream starts to be absorbed by the perovskite-type composite oxide at 200-250°C. Afterwards, when the _CO2 concentration increased rapidly from around 350 °C, the NO concentration started to increase again. Then, the NO concentration starts to show a value almost equal to the inflow concentration of 500 ppm from about 400°C. Because the increase in NO concentration from about 350 °C coincides with the increase in _CO concentration accompanying the combustion of carbon black (simulated PM), it is clear that the NO absorbed by the perovskite-type composite oxide is active in the combustion of carbon black . It is considered that when NO absorbed by the perovskite-type composite oxide is desorbed, a certain substance having strong oxidizing activity (for example, active oxygen) is released, thus causing combustion (oxidation) of carbon black. The ignition point of the carbon black used here is about 560° C., so it can be seen that the perovskite-type composite oxide acts as a catalyst in inducing low-temperature combustion of simulated PM.

The ignition point _T10 was found in the measurement to be the temperature when the amount of _CO2 produced measured in the gas passing through the ventilated fixed bed reached 10% of the total _CO2 production. The results of Example 1 and Comparative Example 1 are listed in Table 2.

Table 1 NO O ₂ H ₂ O N ₂ 500ppm 10% 7% margin

Table 2 composition Ignition point (T ₁₀ ) Example 1 (La _0.8 Sr _0.2 )MnO ₃ 394°C Comparative example 1 Pt/SiO ₂ 430°C

As can be seen from Table 2, the perovskite-type composite oxide according to Example 1 exhibited a lower PM combustion temperature and higher activity than the Pt-impregnated _SiO2 catalyst in Comparative Example 1, even though no noble metal was completely contained. In addition, this perovskite-type composite oxide does not contain a water-soluble component such as potassium (K), so it can be expected to have good durability.

Using a perovskite-type composite oxide that absorbs NO in the temperature range of 200-450°C, PM can be burned at low temperature, so the amount of released PM can be reduced. The generation mechanism of the low-temperature combustion of PM is not yet clear, but the following speculations can be made.

[1] NO in the exhaust gas is oxidized by _O2 in the air through the catalytic action of the perovskite-type composite oxide, and thus absorbed into the perovskite-type composite oxide in the form of _NO2 or nitrate ions.

[2] When these substances are desorbed again, strong oxidizing nitrogen oxides or reactive oxygen species are formed.

[3] The nitrogen oxides or active oxygen thus formed burn PM at low temperature, thereby generating NO and CO ₂ .

Evaluation of PM Combustion Temperature Based on Honeycomb Filter Samples

Various powders obtained in Example 1, Example 2 and Comparative Example 1 were wash-coated into a 200 cpsi cordierite honeycomb used as a DPF. The coating amount is such that the mass of powder is 10 parts when the mass of the honeycomb structure is 100 parts. Then, they were uniformly covered with commercial carbon black as simulated PM. The amount of carbon used is such that the mass of the powder is 2 parts by mass when the mass of the honeycomb structure is 100 parts.

The resulting honeycomb filter samples were installed in ventilated fixed beds, and these beds were brought into contact with a constant flow of simulated diesel engine exhaust as shown in Table 3, so that the _CO2 concentration in the gas passing through the vented fixed beds could be continuously measured. In Table 3, SV is the space velocity expressed by the following equation.

Then, once the simulated diesel exhaust gas flow started, the temperature was raised from room temperature to 750 °C at a heating rate of 10 °C/min while monitoring the _CO2 concentration in the gas passing through the ventilated fixed bed.

CO _{concentration} was detected using a Shimadzu FID-methanizer.

Figure 3 shows an example of _CO2 concentration changes in Examples 1, 2 and Comparative Example 1. It can be seen from Fig. 3 that the increase in _CO2 concentration accompanying the combustion of carbon black (simulated PM) in Examples 1 and 2 occurs at a low temperature of about 300°C. In other words, in the same manner as the previous microparticle samples, it was seen that the perovskite-type composite oxide functions as a catalyst in causing low-temperature combustion of simulated PM.

The ignition point _T10 is shown in Table 4.

table 3 NO O ₂ H ₂ O N ₂ SV 1000ppm 10% 7% margin 20000/h

Table 4 composition Ignition point (T ₁₀ ) Example 1 (La _0.8 Sr _0.2 )MnO ₃ 397°C Example 2 (La _0.8 Sr _0.2 )FeO ₃ 402°C Comparative example 1 Pt/SiO ₂ 478°C

As can be seen from Table 4, the perovskite-type composite oxides according to the present invention (Examples 1 and 2) do exhibit a significant effect of reducing the combustion temperature of PM even though no noble metal is contained at all, and are found to have high practical value. In other words, the catalyst according to the present invention can be expected to provide higher performance and higher reliability than conventional noble metal catalysts in practical use.

Evaluation of catalytic effect after heating to high temperature

The perovskite-type composite oxide obtained after the heat treatment in Example 3 (after heat treatment at 800° C. for 24 hours) was evaluated in the same manner as in FIG. 3 above, and the results are shown in FIG. 4 . For comparison, FIG. 4 also shows the results of the non-heat-treated Pt catalyst (Comparative Example 1) and the non-heat-treated perovskite-type composite oxide (Example 2), for comparison with the case of Example 3.

From the results in Fig. 4, it is clear that compared with the unheated Example 2, the oxide of Example 3 after heat treatment exhibited a slightly lower conversion efficiency from carbon to _CO2 in the low temperature region, but no excessive decrease in activity occurred . In addition, the heat-treated Example 3 oxide retained better activity than the catalytically evaluated unheated conventional Pt catalyst of Comparative Example 1. In other words, the perovskite-type composite oxide according to the present invention maintains catalytic activity even under harsh environments, and enables combustion of particulate matter at lower temperatures than conventional catalysts.

Note that when comparing the ignition points of Example 2 and Comparative Example 2, the ignition point of Example 2 was 358°C and that of Comparative Example 2 was 380°C, so it was confirmed that the catalyst containing sodium had a slightly higher ignition point.

Claims (5)

CLAIMS 1. A diesel exhaust particulate matter oxidation catalyst using a perovskite type composite oxide having an NO absorption region in the range of 200-450°C. 2. The diesel exhaust particulate matter oxidation catalyst according to claim 1, wherein the perovskite type composite oxide can be represented by the structural formula RTO ₃ , wherein R is selected from rare earth elements, alkali metal elements other than Na, and alkaline earth metal elements One or more elements in; and T is one or more elements selected from transition metal elements and Mg, Al and Si. 3. The diesel engine exhaust particulate matter oxidation catalyst according to claim 1, wherein the perovskite composite oxide can be represented by the structural formula RTO ₃ , wherein R includes one selected from La, Sr, Ba, Ca and Li or Multiple elements; T includes one or more elements selected from Mn, Fe, Co, Cu, Zn, Ga, Zr, Mo, Mg, Al and Si. 4. The diesel exhaust particulate matter oxidation catalyst according to any one of claims 1 to 3, wherein, in an exhaust gas atmosphere containing NO, the catalyst induces carbon monoxide in the diesel exhaust gas mainly composed of carbon at a temperature lower than 450°C. Combustion of particulate matter. 5. A particulate matter filter for controlling exhaust emissions from a diesel engine, which is equipped with a diesel exhaust particulate matter oxidation catalyst as claimed in any one of claims 1-4.

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