JP4941975B2 - Method for producing exhaust gas purification catalyst - Google Patents

Description

  The present invention relates to a method for producing an exhaust gas purifying catalyst containing an alkali metal in a catalyst layer.

  As a catalyst for exhaust gas purification of automobiles, an occlusion type lean NOx catalyst is known in which an alkali metal such as potassium (K) is added as a NOx occlusion agent to a catalyst layer to improve NOx occlusion performance. For example, in a vehicle equipped with a diesel engine, a catalyst is supported on a diesel particulate filter (DPF) that captures and removes particulate matter (PM) in exhaust gas, and potassium (K) is added to the catalyst layer. 2. Description of the Related Art A catalyst-supported DPF is known in which an alkali metal such as is added to promote PM (carbon) combustion.

  However, alkali metals such as potassium (K) are highly reactive, and in particular, Si, which is a constituent element of cordierite or silicon carbide (SiC), which is generally used as a support for automobile exhaust gas purification catalysts. There is a problem in that the alkali metal moves into the support and promotes deterioration of the support. Further, there is a problem that alkali metals are easily scattered by aging.

  Therefore, since zeolite has a property of strongly adsorbing alkali metal (see Patent Document 1), zeolite is added to the catalyst layer together with alkali metal, and the alkali metal is adsorbed on the zeolite to thereby support the alkali metal support. Various exhaust gas purifying catalysts that prevent movement and scattering have been developed.

  However, according to recent research, when zeolite is added to the catalyst layer, the migration and scattering of alkali metal to the support can be prevented well, but the NOx occlusion performance in the occlusion type lean NOx catalyst and catalyst supported DPF As a result, it has been found that the effects of improving the PM and promoting the combustion of PM cannot be sufficiently obtained. It has also been found that this tendency becomes more conspicuous as the catalyst temperature increases, particularly in the storage type lean NOx catalyst.

This is because normal zeolite captures some alkali metal in the pores, but PM particles larger than the pore diameter cannot contact the alkali metal in the pores, and the high reactivity of the alkali metal is not exhibited. This is probably because of this. In particular, in the occlusion type lean NOx catalyst, the alkali metal function is required when the catalyst is at a high temperature, but pore clogging occurs due to nitrification by NOx occlusion of the alkali metal at the pore inlet, resulting in pores. It is considered that the diffusion of exhaust gas to the inside becomes difficult.
JP 2004-275814 A

  The present inventors have found that the above problem can be solved by adding a clathrate compound containing an alkali metal to the catalyst layer, and previously filed a patent application (Japanese Patent Application No. 2005-272050).

  By using the clathrate compound that includes the alkali metal, it is possible to prevent the alkali metal from moving to the support and suppress the deterioration of the support, and to prevent the alkali metal from scattering. As described above, since the alkali metal is not buried in the pores, the high reactivity of the alkali metal can be continuously exhibited even in the state where the alkali metal is adsorbed on the host.

  However, it has been found that there is room for further improvement in the initial performance of the catalyst added to the clathrate compound containing the alkali metal.

  Accordingly, an object of the present invention is to improve the initial performance of an alkali metal clathrate compound catalyst.

In order to achieve the above object, according to the present invention, a step of forming a catalyst layer containing a clathrate compound including at least one alkali metal on the surface of a support, ° C. viewing including the step of firing over 0.5 hours at a high temperature of to 900 ° C., is formed by drying after stirring at the clathrate compound 80 ° C. sodalite with an aqueous solution of water-soluble sources of the alkali metal The present invention provides a method for producing an exhaust gas purifying catalyst.

  According to the present invention, an alkali metal clathrate compound catalyst having excellent initial performance is provided.

  Hereinafter, the present invention will be described in more detail with respect to various embodiments.

  The method for producing an exhaust gas purifying catalyst of the present invention includes a step of forming a catalyst layer containing a clathrate compound containing at least one alkali metal on the surface of a support, and the clathrate compound is added at 700 ° C. to A step of baking at a high temperature of 900 ° C.

  The catalyst containing a clathrate compound including an alkali metal according to the present invention can be applied to both a catalyst-supported DPF and an occluded lean NOx catalyst. When applied to a catalyst-carrying DPF, a wall flow monolith support is usually used as the support (filter). The wall flow type monolith support is one in which cells constituting the exhaust gas flow path are alternately sealed, and the PM is filtered by the wall of the support. Further, when applied as an occlusion type lean NOx catalyst, the support is usually a monolith support having tubular cells constituting an exhaust gas flow path in the axial direction, for example, a monolith honeycomb support having a plurality of tubular cells. Used. In any case, the support can be formed of, for example, cordierite, SiC, metal, or the like. In the present invention, deterioration of the support formed of a silicon-containing material such as cordierite or SiC is remarkably suppressed.

  As the alkali metal which is an active ingredient as a NOx storage agent or PM carbon combustion accelerator, at least one of lithium, sodium, potassium and the like can be used, but potassium is particularly preferable from the viewpoint of catalyst performance.

A host material that provides a clathrate compound that includes an alkali metal is made of a material that is porous in terms of molecular weight but does not have openings on the surface, and has a large number of dimple-like holes on the surface. And those containing aluminum as constituent elements are preferred. Examples of such host materials include sodalite (SOD) (eg, Na ₆ [(AlO ₂ ) ₆ (SiO ₂ ) ₆ ] · 2NaX), mesoporous silica (eg, MCM-41, etc.), DDR type, DOH type, MEP type, MSO type, MTN type zeolite and the like. Of these host materials, preferred is sodalite which is excellent in alkali metal holding ability and relatively easily available.

Sodalite has a property that a large number of dimple-like pores on the surface of the molecule constitute a cation exchange point, and alkali metal ions are trapped in the dimple-like pores at the cation exchange point. The central anion X ⁻ of sodalite may be any of hydroxide ion (OH ⁻ ), chloride ion (Cl ⁻ ), bromide ion (Br ⁻ ), etc. Sodalite having ions is preferable in terms of reactivity.

Sodalite can be a commercially available product or can be synthesized. In the synthesis, the silicon (Si) source, the aluminum (Al) source and sodium in an amount providing each oxide having a chemical composition of SiO ₂ /0.5AlO ₂ / 10Na ₂ O / 41H ₂ O, for example, in molar ratio The aqueous mixture containing the (Na) source can be hydrothermally treated. As the Si source, silica, sodium silicate, or the like can be used. As the Al source, sodium aluminate, aluminum hydroxide, or the like can be used. Moreover, sodium hydroxide, sodium chloride, etc. can be used as Na supply source. When chloride is used as the source of each oxide, sodalite having a chloride ion as the central anion is obtained, and when bromide is used, sodalite having a bromide ion as the central anion is obtained. When used, sodalite having a hydroxide ion as the central anion is obtained. The drying process is left overnight at 110 ° C. The sodalite obtained is pulverized after drying.

  In order to include the alkali metal in the host material, the powder of the host material can be mixed with an aqueous solution of a water-soluble alkali metal source, and the mixture can be dried and fired. Examples of the water-soluble alkali metal supply source include alkali metal carbonates. The alkali metal is preferably included in an amount corresponding to 5% to 30% of the weight of the host material.

  Now, in this invention, the clathrate compound which included the alkali metal is baked at the high temperature of 700 to 900 degreeC. The baking at this high temperature can be performed for 0.5 hour or more. By this firing, the excellent carbon combustibility and NOx occlusion property of the clathrate compound are sufficiently exhibited from the beginning of use, and the alkali metal is stabilized so as not to be removed by washing with water. By this high-temperature firing, it is considered that the alkali metal reacts with silicon in the support to form another stable compound.

  Firing at a high temperature can be performed by several methods. In the first technique, the clathrate compound is calcined in advance at the above-mentioned high temperature before the catalyst layer containing the clathrate compound is applied to the support surface.

  In the second method, the clathrate compound before high-temperature firing is applied as a catalyst layer to the surface of the support, and then the entire support is subjected to the above high temperature.

  In any case, the clathrate compound is dispersed in water together with an appropriate amount of binder (for example, alumina sol) to prepare an aqueous slurry, which is applied to a support. Thereafter, the support coated with the slurry is dried. Needless to say, when the clathrate compound has not been previously calcined at the high temperature, the calcining is performed at the high temperature after drying.

  In any case, high-temperature calcination is the first step in the process of producing the exhaust gas purification catalyst of the present invention. Before this high-temperature calcination, the clathrate compound of the present invention is subjected to a high temperature (700 ° C. or higher). It is not a thing. That is, the exhaust gas purifying catalyst of the present invention is provided as a product for the first time after being subjected to this high-temperature firing, and is used for the first time.

  In the present invention, the DPF catalyst layer can contain a three-way catalyst containing a noble metal such as platinum in addition to the clathrate compound of the present invention. In that case, a three-way catalyst in which a noble metal is supported on a porous carrier such as alumina is added to the aqueous slurry, and the resulting aqueous slurry can be treated in the same manner as described above.

  Further, the catalyst layer in the occlusion type lean NOx catalyst can contain an alkaline earth metal such as barium in addition to the clathrate compound of the present invention.

  In the clathrate compound of the present invention, an alkali metal such as potassium is captured as an alkali metal ion at the cation exchange point, for example, in a dimple-like hole of sodalite, and is prevented from moving to a support (eg, cordierite, SiC). As a result, deterioration of the support is suppressed and scattering of alkali metal is prevented.

  The fact that the alkali metal is trapped in the dimple-shaped holes of sodalite means that potassium is trapped on almost the surface of the sodalite, and in the case where PM is burned and removed, it is reactive. Alkaline metal having a high value acts on PM well, and even if the temperature of the catalyst-carrying DPF is relatively low, the PM trapped in the catalyst-carrying DPF can be burned well. As a result, in order to burn PM, it is necessary to supply fuel to the DPF and burn it so that the catalyst-supported DPF has a high temperature, so that the fuel consumption can be reduced and wasteful fuel consumption can be suppressed. be able to. In addition, in the occlusion type lean NOx catalyst, since alkali metal is trapped on almost the surface of sodalite, NOx in the exhaust gas is always adsorbed satisfactorily by the alkali metal that is the NOx occlusion agent regardless of the temperature. Thus, the NOx purification performance is maintained high by being stored in the NOx storage catalyst.

  By firing the clathrate compound having such excellent performance at a high temperature according to the present invention, the performance can be sufficiently exhibited from the beginning.

  EXAMPLES Hereinafter, although an Example demonstrates this invention, this invention is not limited to them.

Examples 1-3
As raw materials for sodalite, sodium silicate (Si source), aluminum hydroxide (Al source), sodium hydroxide (Na source), and water are used, and these raw materials are used as SiO ₂ /0.5AlO ₂ / 10Na _2. Mixing in proportions to provide a chemical composition of O / 41H ₂ O. This mixture was hydrothermally treated at 100 ° C. for 24 hours without stirring, filtered and dried to obtain sodalite and pulverized.

  The obtained sodalite powder was stirred with aqueous solutions of various concentrations of potassium carbonate at 80 ° C. for 24 hours and dried to obtain a sodalite powder containing potassium in a proportion of 1% by weight of the sodalite. It was.

The potassium clathrate sodalite powder thus obtained was calcined at 700 ° C. (Example 1) or 800 ° C. (Example 2) for 5 hours. A sample was prepared by mixing 1 wt% of carbon black powder (manufactured by Mitsubishi Chemical Corporation; grade # 2600; average particle size 13 nm; volatile content 1.8 wt%) with this potassium clad sodalite powder after firing. . In order to measure the carbon combustion temperature of this sample, a thermogravimetric-mass spectrometry (TG-MS) apparatus was used, and m as a combustion product of carbon black while raising the temperature from room temperature to 700 ° C. at a rate of temperature increase of 10 K / min. The amount of carbon dioxide generated at / e = 44 was observed. Further, instead of potassium clathrate sodalite, a similar sample was prepared using the same amount of silicon dioxide (SiO ₂ ) (Example 3), and the amount of carbon dioxide generated was similarly observed.

  The results are shown in FIG. In FIG. 1, the line a shows the result of the sample of Example 1, the line b shows the result of the sample of Example 2, and the line c shows the result of the sample of Example 3.

  From the results shown in FIG. 1, it can be seen that the samples (Examples 1 and 2) using potassium clathrate sodalite have significantly improved carbon combustion characteristics than the samples using silicon dioxide. And the sample (Example 2) which baked potassium clathrate sodalite at a high temperature of 800 ° C. has significantly improved carbon combustion characteristics than the sample (Example 1) baked potassium clathrate sodalite at 700 ° C., It can be seen that the initial performance is excellent.

Examples 4-13
First, Si source, Al source and Na source shown in Table 1 below were used, and these raw materials were mixed at a ratio providing a chemical composition of SiO ₂ /0.5AlO ₂ / 10Na ₂ O / 41H ₂ O. This mixture was hydrothermally treated at 100 ° C. for 24 hours without stirring, filtered and dried to obtain sodalite, which was prepared into powders having various particle sizes shown in Table 1 below.

  The obtained sodalite powder was stirred with aqueous solutions of potassium carbonate of various concentrations at 80 ° C. for 24 hours and dried to obtain sodalite (clathrate compound) containing potassium in the proportions shown in Table 1 below. It was.

The potassium clathrate sodalite powder thus obtained was repeatedly fired at the temperatures shown in Table 1 for 5 hours, as shown in Table 1 below. The calcined sodalite powder after firing was mixed with 1% by weight of the carbon black powder used in Examples 1 to 3 to prepare a sample. In order to measure the carbon combustion temperature of this sample, a thermogravimetric-mass spectrometry (TG-MS) apparatus was used, and m as a combustion product of carbon black while raising the temperature from room temperature to 700 ° C. at a rate of temperature increase of 10 K / min. The amount of carbon dioxide generated at / e = 44 was observed, and the combustion start temperature (T _ig ) of carbon black was examined. This is also shown in Table 1 below. Table 1 below also lists a value (−ΔT _ig ) obtained by subtracting the carbon combustion start temperature in each sample from the combustion start temperature (520 ° C.) of the carbon itself.

  From the results shown in Table 1, it can be seen that the samples of Examples 6 to 13 have excellent initial performance.

The graph which shows the carbon combustion characteristic of a baking potassium clathrate sodalite with a comparative example.

Claims (5)

Forming a catalyst layer containing a clathrate compound containing at least one alkali metal on the surface of the support, and calcining the clathrate compound at a high temperature of 700 ° C. to 900 ° C. for 0.5 hour or more . step only contains, producing an exhaust gas purifying catalyst, wherein the clathrate compound is intended to be formed by drying after stirring at 80 ° C. sodalite with an aqueous solution of water-soluble sources of the alkali metal Method. The method for producing an exhaust gas purifying catalyst according to claim 1, wherein the alkali metal is potassium. The method for producing an exhaust gas purifying catalyst according to claim 1 or 2 , wherein the catalyst layer further includes a three-way catalyst. After firing advance in the hot the clathrate compound, according to any one of claims 1 and forming a layer of a catalyst containing a clathrate compound forms該焼the surface of the support 3 Of manufacturing exhaust gas purification catalyst. The method for producing an exhaust gas purifying catalyst according to any one of claims 1 to 3 , wherein after the catalyst layer is formed on the surface of the support, the catalyst layer is calcined at the high temperature. .

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