JP2006043637A - Catalyst for cleaning exhaust gas - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a catalyst for cleaning an exhaust gas which suppresses poisoning by sulfur and suppresses sintering of a catalytic noble metal. <P>SOLUTION: In the catalyst for cleaning the exhaust gas prepared by carrying a NOx occlusion agent selected from a group comprising a alkaline earth metal and a rare earth element, and an noble metal for a catalyst on a carrier comprising titania-zirconia-alumina powders, the carrier comprising the titania-zirconia-alumina powders is porous and the volume of pores with a diameter of 20 nm or less is 104 cc/g or more. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an exhaust gas purifying catalyst for purifying exhaust gas discharged from an internal combustion engine such as an automobile, and more particularly, a highly durable exhaust gas purifying catalyst in which sintering of catalyst components in a high temperature lean atmosphere is remarkably suppressed. About.

  Conventionally, as a catalyst for exhaust gas purification of automobiles, a three-way catalyst that purifies by performing CO and HC oxidation and NOx reduction simultaneously in exhaust gas at a stoichiometric air-fuel ratio (stoichiometric) has been used. As such a three-way catalyst, for example, a porous carrier layer made of γ-alumina is formed on a heat-resistant substrate made of cordierite or the like, and platinum (Pt), rhodium (Rh) or the like is formed on the porous carrier layer. A catalyst on which a catalyst noble metal is supported is widely known.

On the other hand, in recent years, from the viewpoint of protecting the global environment, carbon dioxide (CO ₂ ) in exhaust gas discharged from internal combustion engines such as automobiles has been a problem, and so-called lean burn that makes lean combustion in an oxygen-excess atmosphere is promising as a solution. Is being viewed. In this lean burn, since the fuel consumption is improved, the use of fuel is reduced, and the generation of CO _{2 as} the combustion exhaust gas can be suppressed.

  On the other hand, the conventional three-way catalyst is one that simultaneously oxidizes, reduces, and purifies CO, HC, NOx in the exhaust gas when the air-fuel ratio is the stoichiometric air-fuel ratio (stoichiometric). In an oxygen-excess atmosphere of the exhaust gas at that time, sufficient purification performance is not shown for NOx reduction and removal. Therefore, development of a catalyst and a purification system that can purify NOx even in an oxygen-excess atmosphere is desired.

  Therefore, the applicant of the present application has previously proposed an exhaust gas purification catalyst in which an alkaline earth metal and Pt are supported on a porous carrier such as alumina, and an exhaust gas purification catalyst in which lanthanum and Pt are supported on a porous carrier. According to these exhaust gas-purifying catalysts, NOx is occluded in the alkaline earth metal oxide or lanthanum oxide (NOx occlusion material) on the lean side, which is generated in the transient region, or HC or NO on the rich side. Since it is purified by reacting with a reducing component such as CO, the NOx purification performance is excellent even on the lean side.

  By the way, the exhaust gas contains SOx produced by burning sulfur (S) contained in the fuel. This SOx is oxidized by the catalytic metal on the lean side, and also reacts with water vapor to generate sulfite ions and sulfate ions. And it became clear that when these react with the NOx occlusion material to produce sulfites and sulfates, the NOx occlusion action of the NOx occlusion material is impaired and the purification performance deteriorates.

  In the conventional exhaust gas purifying catalyst, activated alumina having excellent adsorption action is used as a carrier. However, since the activated alumina carrier has a property of easily adsorbing SOx, the sulfur poisoning is promoted. There was also a phenomenon. Further, when SOx is adsorbed by alumina, the alumina becomes acidic and repels NOx, and the NOx occlusion in the NOx occlusion material is inhibited. Furthermore, SOx and NOx occlusion material may produce sulfate or sulfite, and since this sulfate or sulfite is difficult to decompose, the NOx occlusion action of the NOx occlusion material cannot be recovered and the durability is impaired. there were.

  Accordingly, the applicant of the present application has proposed an exhaust gas purification catalyst in which a NOx storage material and a catalyst noble metal are supported on a support made of a composite oxide of Ti and Zr. According to this exhaust gas-purifying catalyst, the composite oxide of Ti and Zr is less likely to adsorb sulfate ions and sulfite ions than alumina, and even if it is adsorbed and becomes sulfate, the sulfate is easily decomposed. And the combined stabilization of Ti and Zr has the effect of improving heat resistance and acidity, and is effective in improving catalyst performance and preventing sulfur poisoning.

However, as a result of further research, the exhaust gas purification catalyst using a support made of a composite oxide of Ti and Zr has a large thermal deterioration when used at a high temperature of 900 ° C. or higher, and the NOx purification rate after high temperature durability is high. It became clear that the degree of decline was large. Therefore, with the aim of preventing sulfur poisoning while maintaining high NOx purification performance and improving the heat resistance and reducing the degree of decrease in NOx purification rate after high temperature durability, titania (TiO ₂ ) -zirconia ( It has been proposed to use ZrO ₂ ) powder as a support in combination with alumina (Al ₂ O ₃ ) powder (see Patent Document 1).

JP 2000-327329 A

By the way, for example, in the case of an exhaust gas purification catalyst for an automobile engine, a reducing atmosphere in which the temperature repeatedly fluctuates between room temperature and about 1000 ° C., and the HC and CO concentrations are relatively high and the O ₂ concentration is low. It is necessary to maintain a highly dispersed support state of the catalyst noble metal under conditions where an oxidizing atmosphere having relatively low HC and CO concentrations and high O ₂ concentration is repeated.

  However, the above-mentioned catalytic noble metal has a property of causing so-called sintering, when the catalytic noble metal is exposed to such an atmosphere for a long period of time, and moves on the support to form enlarged particles. For this reason, a catalyst noble metal cannot maintain a high contact area with exhaust gas, and there exists a problem that the purification performance of exhaust gas falls with time.

  The present invention provides an exhaust gas purifying catalyst using a carrier comprising such titania-zirconia-alumina powder, which suppresses sintering of the catalyst noble metal and has excellent durability without deterioration of the catalyst performance even in a high temperature atmosphere. An object is to provide a purification catalyst.

  In order to solve the above problems, according to the present invention, a NOx occlusion material selected from the group consisting of an alkali metal, an alkaline earth metal and a rare earth element and a catalyst noble metal are supported on a support made of titania-zirconia-alumina powder. In the exhaust gas purifying catalyst, the support made of the titania-zirconia-alumina powder is porous and the capacity of pores having a diameter of 20 nm or less is 0.4 cc / g or more. A catalyst is provided.

  The exhaust gas purifying catalyst of the present invention can suppress sulfur poisoning and improve heat resistance by constituting the carrier for supporting the catalyst noble metal from titania-zirconia-alumina powder. Furthermore, in this carrier, sintering of the catalyst noble metal can be prevented by setting the capacity of the pores having a diameter of 20 nm or less to 0.4 cc / g or more.

  The exhaust gas purifying catalyst of the present invention comprises a support made of titania-zirconia-alumina powder and a NOx occlusion material selected from the group consisting of alkali metals, alkaline earth metals and rare earth elements and a catalyst noble metal supported thereon. In the present invention, the titania-zirconia-alumina powder means a mixture of titania powder, zirconia powder and alumina powder, a mixture of titania-zirconia solid solution powder and alumina powder, and titania-zirconia-alumina solid solution powder.

  In this titania-zirconia-alumina powder, the ratio of Ti, Zr and Al is not particularly limited, but Ti and Zr are in the range of the molar fraction of Zr represented by Zr / (Ti + Zr) in the range of 0.2 to 0.5. It is preferable that This is because if the molar fraction is out of this range, the specific surface area of the carrier cannot be reduced, and improvement in acidity (number of acid sites) cannot be expected.

  Further, Al is preferably in the range of 1 to 9 in molar ratio with respect to the total of Ti and Zr. When Al is less than this range, the heat resistance is not sufficient, and when it exceeds this range, sulfur poisoning tends to occur. The carrier can be coated on the surface of the monolith carrier substrate, metal carrier substrate or pellet substrate as a carrier layer. Moreover, you may form a monolith support base material and a pellet support base material from support | carrier itself.

  In the present invention, in the titania-zirconia-alumina powder constituting the carrier, the pore volume having a diameter of 20 nm or less needs to be 0.4 cc / g or more. In the present specification, the diameter and volume of the pores are values measured by a mercury porosimetry porosimeter.

  As shown in FIG. 1, the catalyst noble metal (for example, Pt) is supported not only on the surface of the support but also in the pores. When this catalyst is exposed to high temperatures, Pt migrates on the support surface to form enlarged particles. On the other hand, Pt supported in so-called mesopores having a diameter of 2 to 20 nm is limited by the size of the pores and is prevented from becoming larger than 20 nm. From our experimental results, it has been found that a sufficient capacity of NOx occlusion can be obtained by setting the capacity of pores having a diameter of 20 nm or less to 0.4 cc / g or more.

  For this titania-zirconia-alumina powder, for example, a raw material solution containing a titanium salt, a zirconium salt, and an aluminum salt is prepared at a predetermined quantitative ratio, and a pH adjuster is added while stirring the raw material solution to form a precipitate. The precipitate obtained is dried and fired. Here, it is preferable to add hydrogen peroxide and a surfactant to the raw material solution. This is because, by adding hydrogen peroxide, titanium ions, zirconium ions, and aluminum ions become complex ions, and the pH at which these precipitates are formed becomes close, so that the coprecipitate is easily mixed at the atomic level. In addition, even if a non-uniform portion is generated during the precipitation operation due to the addition of the surfactant, particles mainly composed of zirconia, particles mainly composed of titania, and alumina are mainly contained in the micelles of the surfactant. This is because the particles as the components grow in the direction in which the composition becomes uniform, and solid solution particles having a uniform composition are easily formed.

  As the surfactant, any of an anionic system, a cationic system, and a nonionic system can be used. In particular, it is preferable to use a surfactant that can form a narrow space on the inner surface of the micelle to be formed, for example, a spherical micelle. Further, it is preferable to use a surfactant having a critical micelle concentration of 0.1 mol / liter or less. The critical micelle concentration means the lowest concentration at which a certain surfactant forms micelles.

  Various conventional methods can be used to make the carrier porous. For example, the raw material solution containing the surfactant is stirred and fired in a state where bubbles are mixed in the raw material solution. Before firing, bubbles exist in the carrier, and the bubble portions become pores by firing. By adjusting the amount and size of the bubbles, the capacity of pores having a diameter of 20 nm or less in the carrier can be set to 0.4 cc / g or more. It is also possible to form pores by mixing carbon powder, wood powder, resin powder or the like with a controlled particle size into the raw material solution and burning off the carbon powder or the like by firing. By adjusting the particle size and addition amount of the carbon powder or the like, the capacity of pores having a diameter of 20 nm or less in the carrier can be set to 0.4 cc / g or more.

As the NOx occlusion material supported on the carrier, at least one selected from alkali metals, alkaline earth metals and rare earth elements is used. Examples of the alkali metal include lithium, sodium, potassium, and cesium. Moreover, alkaline earth metal means a periodic table 2A group element, and magnesium, calcium, strontium, and barium are mentioned. Examples of rare earth elements include scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, and the like. The content of the NOx storage material is preferably in the range of 0.05 to 1.0 mol with respect to 100 g of the carrier. Content is less than 0.05 mol and _the NO _x storage capacity NOx purification performance is decreased smaller, also she contains more than 1.0 mole, NOx storage capacity of such emissions and simultaneously saturated HC increases A malfunction occurs.

  As the catalyst noble metal, one or more of Pt, Rh and Pd can be used, and Pt is particularly desirable. The supported amount of any precious metal is preferably 0.1 to 20 g, particularly preferably 0.5 to 10 g, per 100 g of support (corresponding to a volume of 1 liter of the entire catalyst). Even if the amount of the catalyst noble metal supported is increased further, the activity is not improved, and the effective utilization cannot be achieved. On the other hand, if the amount of the catalyst noble metal supported is less than this, practically sufficient activity cannot be obtained.

  In order to support the NOx occlusion material and the catalyst noble metal on the carrier, it can be supported in the same manner as before using the impregnation method, spraying method, slurry mixing method, etc., using the chloride or nitrate thereof.

Examples 1-6 and Comparative Examples 1-6
Aluminum nitrate, zirconium oxynitrate, and titanium tetrachloride are added to 1000 mL of water in the ratio shown in Table 1 below, and mixed by stirring. 0.01 mol of a surfactant (Leocon 1020H, manufactured by Lion Corporation) is added to this mixed solution. A raw material aqueous solution was prepared. While stirring this raw material aqueous solution, 25% ammonia was added to adjust the pH to 8.8. This aqueous solution was left to mature at 120 ° C. and 2 atm for 2 hours, and the resulting precipitate was separated by centrifugation and calcined at 500 ° C. for 2 hours to obtain a carrier powder. In Comparative Example 1, no aging step was performed.

  With respect to the obtained carrier powder, the pore diameter and pore volume were measured with a mercury porosimetry porosimeter, and the volume of pores having a diameter of 20 nm or less is shown in Table 1.

  A predetermined amount of the carrier powder was immersed in a dinitrodiammine platinum aqueous solution having a predetermined concentration, stirred for 5 hours, evaporated to dryness, and baked in the atmosphere at 500 ° C. for 2 hours to carry Pt. The amount of Pt supported is 2 g of Pt with respect to 100 g of carrier (corresponding to 1 L). Next, the carrier powder carrying Pt is immersed in an aqueous barium acetate solution having a predetermined concentration, stirred for 5 hours, evaporated to dryness, and calcined in the atmosphere at 500 ° C. for 2 hours to obtain a NOx occlusion material. Ba was supported. The amount of Ba supported is 0.2 mol of Ba with respect to 100 g of carrier (corresponding to 1 L).

  Finally, the carrier powder carrying Pt and Ba was treated in a hydrogen stream at 500 ° C. for 3 hours and pelletized to obtain an exhaust gas purification catalyst.

(Examination / Evaluation)
With respect to the exhaust gas purifying catalyst, the NOx occlusion amount after the sulfur poisoning durability test was measured. In the sulfur poisoning endurance test, 1 g of each catalyst was placed in an endurance test device, and the temperature was raised from room temperature to 600 ° C. in 30 minutes while alternately repeating the rich and lean model gases shown in Table 2 every 30 seconds. This was done by holding at 600 ° C. for 4 hours.

  The NOx occlusion amount was measured after the above sulfur poisoning endurance test was performed, and each catalyst was packed in a fixed bed flow type reaction tube, and the gas composition shown in Table 3 at a temperature of 300 ° C. was first set to a rich condition. After leaning, the NOx occlusion amount until the outlet gas NOx concentration reached the inlet gas NOx concentration was evaluated.

  The results are shown in Table 4 below and FIG. Moreover, this result is shown in FIG.3 and FIG.4 by the relationship between alumina content and zirconia content.

  As shown in FIG. 2, it can be seen that when the pore volume with a diameter of 20 nm or less is 0.4 cc / g or more, a good NOx occlusion amount is achieved.

It is a conceptual diagram explaining the effect | action of the catalyst for exhaust gas purification of this invention. It is a graph which shows the relationship between the pore capacity | capacitance of diameter 20nm or less, and NOx occlusion amount. It is a graph which shows the relationship between the alumina content in a support | carrier, and NOx occlusion amount. It is a graph which shows the relationship between zirconia content in a support | carrier, and NOx occlusion amount.

Claims (1)

  In the exhaust gas purifying catalyst in which a NOx storage material selected from the group consisting of an alkali metal, an alkaline earth metal and a rare earth element and a catalyst noble metal are supported on a support made of titania-zirconia-alumina powder, the titania-zirconia- A catalyst for purification of exhaust gas, wherein the support made of alumina powder is porous and the capacity of pores having a diameter of 20 nm or less is 0.4 cc / g or more.

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