JP2005095761A - Exhaust gas purification catalyst - Google Patents

Abstract

[PROBLEMS] To provide a catalyst capable of reliably preventing coking and exhibiting high activity as a three-way catalyst using zeolite.
SOLUTION: It is composed of zeolite particles 1 supporting Rh11 and a porous oxide layer 2 formed on the surface of the zeolite particles and supporting Pt20.
Since Pt and Rh are supported separately from each other, mutual solid solution can be suppressed. Coking is reliably prevented by the oxidation of carbon by Rh supported in the pores.
[Selection] Figure 2

Description

  The present invention relates to an exhaust gas purifying catalyst suitable for a three-way catalyst for purifying exhaust gas from an automobile.

A three-way catalyst is used as an exhaust gas purifying catalyst for purifying HC, CO and NO _x in exhaust gas from automobiles. This three-way catalyst is formed by supporting a noble metal such as Pt and Rh on a porous oxide such as alumina or ceria, and can simultaneously purify HC, CO and NO _x in an atmosphere near stoichiometric.

  However, when the exhaust gas temperature is in a low temperature range such as when the engine is started, harmful components are discharged without being purified until the precious metal is heated to its activation temperature (until the catalyst is warmed up). There is a problem that. In general, about 80% or more of harmful components discharged during the emission measurement mode are discharged until the exhaust gas purification catalyst is warmed up, and it is an important issue to control the amount of discharge during that time. Yes.

  In particular, since the temperature of the exhaust gas purification catalyst greatly affects the purification of HC, it is a problem to improve the HC purification rate in a low temperature range. Therefore, it has been proposed to use HC adsorbents such as zeolite to suppress emissions by adsorbing HC to HC adsorbents at low temperatures and purify HC released from HC adsorbents with a catalyst after warm-up. ing. For example, Japanese Patent Laid-Open No. 2002-361048 describes a catalyst comprising a lower layer made of HC adsorbent and an upper layer made of a three-way catalyst formed on the surface of the lower layer. Japanese Patent Application Laid-Open No. 2000-024515 describes a catalyst in which a catalyst support layer is formed on a portion excluding the inside of zeolite pores.

  These catalysts have a problem that carbon is produced from HC adsorbed on the HC adsorbent, and the pores of the HC adsorbent are blocked by the carbon, so that the HC adsorbing ability is lowered. In other words, zeolite has acid sites, which are active sites for the reaction, but in the oxygen-deficient state, HC decomposes into carbon and hydrogen at the acid sites, and the generated carbon is called coking that covers the active sites. Occurs. This problem can be solved by supporting a noble metal on the HC adsorbent. This is because if the supported noble metal is heated to the activation temperature, the carbon can be oxidized and decomposed, and the active site is restored. Zeolite also generates heat when adsorbing HC, and this heat also contributes to warming up the noble metal.

  However, in the catalyst described in JP-A-2002-361048, the thickness of the upper layer made of a three-way catalyst is thick, so that HC hardly reaches the lower HC adsorbent layer. In addition, since oxygen gas does not easily reach the lower HC adsorbent layer, there is also a problem that even if a noble metal is supported on the HC adsorbent layer, the effect is not sufficiently exhibited.

By the way, when the exhaust gas purification catalyst began to be used, an oxidation catalyst system was used that suppresses NO _x emissions by adjusting the engine and purifies HC and CO with the catalyst. This oxidation catalyst was one in which Pt was supported on Al ₂ O ₃ and utilized high oxidation activity of Pt. Later, a three-way catalyst supporting Pt and Rh was developed, and a method of purifying HC, CO and NO _x simultaneously by controlling the exhaust gas to a stoichiometric atmosphere has become the mainstream. In this three-way catalyst oxidizes HC and CO primarily by Pt, and by reducing NO _x mainly Rh.

However, a catalyst in which Pt and Rh are supported in a coexistence has a problem that a reaction between metals occurs at a high temperature, and the activity decreases due to solid solution of Pt and Rh. Especially for the loading amount of Pt is greater than the loading amount of Rh in three-way catalysts, Pt Rh is dissolved the activity of Rh is lost, reducing activity of the NO _x is lowered. Further, even if the same kind of precious metals are supported in close proximity, grain growth occurs at a high temperature, and there is a problem that the activity decreases due to a decrease in active sites.

  In view of this, JP-A-01-236942, JP-A-10-216514 and the like disclose exhaust gas purifying catalysts in which Pt and Rh are supported on different oxide carriers. An exhaust gas purification catalyst having a two-layered catalyst layer in which a catalyst layer supporting Pt and a catalyst layer supporting Rh are laminated is also known. By separating and supporting Pt and Rh in this way, it is possible to suppress the solid solution of Pt and Rh from each other, thereby preventing a decrease in activity.

However, the catalyst described in Japanese Patent Laid-Open No. 2000-024515 described above does not describe the technical idea of separating and supporting Pt and Rh. Therefore, when the catalyst described in JP 2000-024515 A is used as a three-way catalyst and noble metal is supported to prevent coking of the zeolite as described above, Pt and Rh are closely supported on the zeolite. As a result, there is a problem that Pt and Rh are dissolved in each other.
JP 2002-361048 JP 2000-024515 JP-A-10-216514

  This invention is made | formed in view of such a situation, and makes it the problem which should be solved to make a catalyst which can prevent coking reliably using zeolite and can express high activity as a three-way catalyst.

  A feature of the exhaust gas purifying catalyst of the present invention that solves the above-mentioned problem is that it comprises a zeolite particle supporting a first noble metal and a porous oxide layer formed on the surface of the zeolite particle and supporting a second noble metal. is there.

  It is desirable that the first noble metal is Rh and the second noble metal is Pt. The porous oxide layer is preferably an alumina layer.

  According to the exhaust gas purifying catalyst of the present invention, since the porous oxide layer supporting the second noble metal is thin, HC and oxygen easily reach the zeolite. Therefore, it excels in HC adsorption ability and HC purification ability. And since the 1st noble metal is carry | supported by the zeolite, when HC adsorb | sucked to the zeolite is discharge | released, the probability of contacting with a 1st noble metal is high, and high HC purification ability is expressed. In addition, the oxidation of carbon by the first noble metal supported in the pores prevents the caulking reliably, and the first and second noble metals are separated by the porous oxide layer. It is suppressed. Therefore, it is extremely excellent in durability.

  In the exhaust gas purifying catalyst of the present invention, a first noble metal is supported on a core made of zeolite, and a second noble metal is supported on a thin-film porous oxide layer on the surface thereof. Therefore, since the first noble metal and the second noble metal are supported separately from each other by the porous oxide layer, when the first noble metal and the second noble metal are of the same type, grain growth at high temperatures is suppressed. be able to. In addition, when Rh and Pt are supported separately, mutual solid solution can be suppressed. Therefore, the decrease in activity is suppressed. Coking is reliably prevented by the oxidation of carbon by the first noble metal supported in the pores.

  Moreover, since the porous oxide layer supporting the first noble metal is thick enough to cover the surface of the zeolite particles, HC and oxygen easily reach the zeolite. Therefore, it has excellent HC adsorption ability and HC purification ability. The first noble metal and the second noble metal are very close to each other even though they are supported by the porous oxide layer. As a result, the respective catalytic reactions can be efficiently communicated and the reaction heat can be easily transferred, so that high activity is exhibited from a low temperature range.

That is, in the low temperature range, HC in the exhaust gas easily passes through the porous oxide layer and is adsorbed on the zeolite. On the other hand, the CO in the exhaust gas is easily oxidized even at low temperatures by the second noble metal supported on the porous oxide layer, and the heat generation is easily transferred to the zeolite, thus promoting the warm-up of the entire catalyst. . When the first noble metal exceeds the activation temperature, the HC adsorbed on the zeolite or the HC released from the zeolite is oxidized and purified by the first noble metal. NO _x is reduced and purified by reacting with CO, HC, or HC released from zeolite in the exhaust gas by the catalytic action of the first noble metal or the second noble metal.

As the zeolite, ZSM-5, mordenite, Y-type zeolite, ferrierite and the like can be used. The SiO ₂ / Al ₂ O ₃ ratio is not particularly limited, but is preferably in the range of 10 to 2000.

  It is desirable to use zeolite particles having an average particle size of 10 μm or less. When the average particle diameter exceeds 10 μm, the surface area is too small, the HC adsorption ability is insufficient, and it may be difficult to form the coat layer.

  The thin porous oxide layer is preferably as thin as possible. By doing so, the heat of the oxidation reaction in the porous oxide layer is more easily transmitted to the zeolite particles, and the reaction is promoted. In order to form such a thin porous oxide layer, it can be formed by an alkoxide method, a precipitation method from a nitrate aqueous solution, or the like, but it is desirable to form the thin porous oxide layer by decomposing the coupling agent. The coupling agent generally has one hydrophilic group per molecule, and the hydrophilic group is chemically bonded to the surface of the zeolite particles, which is an inorganic substance. By baking the coupled coupling agent, the metal atoms contained in the coupling agent become oxides and are formed in a thin film on the surface of the zeolite particles. The porous oxide layer formed in this manner is extremely thin and covers the zeolite particles as an almost monomolecular film on the order of nm.

  As the porous oxide of the porous oxide layer, various porous oxides such as alumina, zirconia, and titania can be used, and alumina is particularly preferable. Zeolite deteriorates due to de-Al, but if an alumina layer is formed on the surface, Al is reinserted into the zeolite from the alumina layer as described in, for example, “Catalyst Conversion Technique” vol. 17 (2000) P21. Therefore, deterioration of the zeolite particles can be suppressed.

As the first noble metal supported on the zeolite particles, at least one of Pt, Pd, Rh, Ir and the like can be used. In particular, it is desirable to carry Rh. The zeolite carrying Rh promotes the reaction of generating H ₂ from HC, for example, as shown in the following formula. Therefore, H ₂ is generated in the exhaust gas, and this reduces NO _x , so that the NO _x purification ability is improved.

CH ₄ + CO ₂ → 2CO + 2H ₂
As the second noble metal supported on the porous oxide layer, at least one of Pt, Pd, Rh, Ir and the like can be used. In particular, it is desirable to carry Pt having excellent ternary activity. If Rh is supported as the first noble metal, Pt and Rh are separated from each other and close to each other, so that solid solution of Rh in Pt can be surely prevented, and the three-way catalyst is high. The activity can be maintained for a long time. Further, since carbon is oxidized by at least one of Pt and Rh supported in the pores, coking can be surely prevented and deterioration of HC adsorption ability can be prevented.

  In the case of Pt and Pd, the supported amount of the noble metal is preferably 0.1 to 20.0 g, particularly preferably 0.5 to 10.0 g with respect to 120 g of the catalyst particles. In the case of Rh, 0.01 to 80 g is preferable and 0.05 to 5.0 g is particularly preferable with respect to 120 g of catalyst particles. In terms of 1 liter of catalyst volume, 0.1 to 20 g is preferable in the case of Pt and Pd, and 0.5 to 10 g is particularly preferable. In the case of Rh, 0.01 to 10 g is preferable, and 0.05 to 5 g is particularly preferable.

  Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples.

(Example 1)
12 g of ZSM-5 powder was mixed in a predetermined amount of an aqueous rhodium nitrate solution having a predetermined concentration, filtered after stirring for 2 hours, dried at 120 ° C. and calcined at 500 ° C. to carry Rh. The amount of Rh supported is 0.05 g.

Next, 36 g (3.7 g as Al ₂ O ₃ ) of acetoalkoxyaluminum diisopropylate (“Plenact AL-M” manufactured by Ajinomoto Finetechno), which is an Al-based coupling agent, is dissolved in n-propanol. The whole amount of Rh / ZSM-5 powder prepared in (1) was added and stirred. This was transferred to a milling vessel, and an appropriate number of ceramic balls were added and milled for 3 hours.

Thereafter, the solvent was separated by decantation and centrifugation. This was repeated three times to remove unreacted coupling agent and alcohol, and after drying, calcination was carried out at 500 ° C. for 2 hours. As a result, the organic group of the coupling agent adhered and fixed to the Rh / ZSM-5 particles is oxidatively decomposed, and Al ₂ O ₃ / Rh / ZSM-5 having a thin Al ₂ O ₃ layer on the surface of the Rh / ZSM-5 particles. A catalyst powder composed of particles was prepared.

The total amount of the resulting Al ₂ O ₃ / Rh / ZSM-5 catalyst powder is mixed with a predetermined amount of dinitrodiammine platinum aqueous solution having a predetermined concentration, filtered after stirring for 2 hours, dried at 120 ° C, and calcined at 500 ° C. Pt was supported. The amount of Pt supported is 0.2 g. Thus, a Pt / Al ₂ O ₃ / Rh / ZSM-5 catalyst powder was prepared.

The obtained catalyst is a powder in which Pt / Al ₂ O ₃ / Rh / ZSM-5 catalyst particles shown in FIGS. 1 and 2 are assembled. The catalyst particles are composed of ZSM-5 particles 1 as a core portion and an Al ₂ O ₃ layer 2 formed on the surface of the ZSM-5 particles 1, and Rh11 is supported on the ZSM-5 particles 1, and Al ₂ Pt20 is supported on the O ₃ layer 2. Since the Al ₂ O ₃ layer 2 is extremely thin, Pt20 and Rh11 are supported in close proximity while being separated from each other.

  The catalyst powder was pelletized by a conventional method and subjected to a test.

(Comparative Example 1)
36 g (3.7 g as Al ₂ O ₃ ) of acetoalkoxyaluminum diisopropylate (“Plenact AL-M” manufactured by Ajinomoto Fine Techno), an Al coupling agent, was dissolved in n-propanol. 12 g of the same ZSM-5 powder was added and stirred. This was transferred to a milling vessel, and an appropriate number of ceramic balls were added and milled for 3 hours.

Then, the process which isolate | separates a solvent by decantation and centrifugation was repeated 3 times, the unreacted coupling agent and alcohol were removed, and it baked at 500 degreeC for 2 hours after drying. As a result, the organic group of the coupling agent adhered and fixed to the ZSM-5 particles is oxidatively decomposed to prepare a powder composed of Al ₂ O ₃ / ZSM-5 particles having a thin Al ₂ O ₃ layer on the surface of the ZSM-5 particles. did.

The obtained Al ₂ O ₃ / ZSM-5 powder was mixed in a predetermined amount of an aqueous rhodium nitrate solution having a predetermined concentration, filtered after stirring for 2 hours, dried at 120 ° C., and calcined at 500 ° C. to carry Rh. Further, it was mixed in a predetermined amount of a dinitrodiammine platinum aqueous solution having a predetermined concentration, filtered after stirring for 2 hours, dried at 120 ° C., calcined at 500 ° C., and supported with Pt. The amount of Rh and Pt supported is the same as in Example 1.

  The obtained catalyst powder was pelletized by a conventional method and subjected to a test.

(Comparative Example 2)
12 g of ZSM-5 powder was mixed in a predetermined amount of an aqueous rhodium nitrate solution having a predetermined concentration, filtered after stirring for 2 hours, dried at 120 ° C. and calcined at 500 ° C. to carry Rh. The amount of Rh supported is 0.05 g.

On the other hand, 3.7 g of Al ₂ O ₃ powder was mixed in a predetermined amount of a dinitrodiammine platinum aqueous solution having a predetermined concentration, stirred for 2 hours, filtered, dried at 120 ° C., and calcined at 500 ° C. to carry Pt. The amount of Pt supported is 0.2 g.

The total amount of the Rh / ZSM-5 powder prepared above and the total amount of the Pt / Al ₂ O ₃ powder were mixed well, pelletized by a conventional method, and used for the test.

<Test and evaluation>
The same amount of each of the pellet catalysts of Example 1 and Comparative Examples 1 and 2 was filled in the evaluation apparatus, and the endurance treatment was performed in which the model gas shown in Table 1 was entered and circulated for 5 hours at a gas temperature of 750 ° C and a flow rate of 5 L / min.

After the endurance treatment, the catalyst bed was kept at room temperature (25 ° C), and the model gas shown in Table 1 was maintained at 400 ° C and flowed through the bypass circuit, then switched to the catalyst bed side at a flow rate of 5 L / min. The catalyst outgas was monitored for 3 minutes. The purification rate of HC, CO and NO _x for 3 minutes was calculated, and the results for each catalyst are shown in FIG.

  From FIG. 3, it is clear that the catalyst of Example 1 has a higher purification activity in 3 minutes from the start than the catalysts of Comparative Examples 1 and 2 even after the endurance treatment. This is considered to be due to the synergistic effect that coking of the pores was suppressed during the durability test and that the decrease in activity due to solid solution was suppressed because Pt and Rh were separated and supported. .

The exhaust gas purifying catalyst of the present invention is effective when used as an exhaust gas purifying catalyst for automobiles, particularly as a three-way catalyst, and is excellent in warm-up performance and efficiently removes HC, CO and NO _x from a low temperature range such as at the start. Can be purified. And since coking is prevented and deterioration, such as solid solution of noble metals, is also prevented, it is excellent in durability.

It is typical sectional drawing which shows the catalyst particle of one Example of this invention. It is typical sectional drawing which shows the catalyst particle of one Example of this invention. HC of each catalyst is a bar graph showing the purification rates of CO and NO _x.

Explanation of symbols

1: ZSM-5 particles 2: Al ₂ O ₃ layer 10: pores

Claims (3)

  An exhaust gas purifying catalyst comprising: zeolite particles supporting a first noble metal; and a porous oxide layer formed on the surface of the zeolite particles and supporting a second noble metal.   The exhaust gas-purifying catalyst according to claim 1, wherein the first noble metal is rhodium, and the second noble metal is platinum.   The exhaust gas purification catalyst according to claim 1 or 2, wherein the porous oxide layer is an alumina layer.

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