JP2006297235A - MANUFACTURING METHOD OF CATALYST HAVING NOx OCCLUDING MATERIAL - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To manufacture an NOx removing catalyst having good regeneration performance from SOx poisoning. <P>SOLUTION: The method is used in manufacturing a catalyst having an NOx occluding material in which at least one compound selected from alkali, alkaline earth and rare earth metals is carried on a carrier as an NOx occluding material and comprises the steps of converting, with an organic acid, the compound of the metal element to serve as the NOx occluding material into a complex ion of a potential opposite to that of the carrier, causing the carrier to carry the complex ion and drying and baking the resultant carrier. The organic acid may be succinic, malic, tartaric or citric acid. The method keeps the NOx occluding material to be kept in a highly dispersed state in the form of particulate throughout the process from the synthesis of the NOx occluding material to the carrying on the carrier, resulting in improvement in regeneration performance from SOx poisoning of the resultant catalyst and thus providing a catalyst of an excellent NOx removing capacity. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for producing a catalyst in which a NOx occlusion material is supported on predetermined carrier particles made of an oxide or a composite oxide.

As a catalyst for purifying combustion exhaust gas, a catalyst carrying a NOx storage material is known. For example, an internal combustion engine such as a gasoline engine or a diesel engine produces exhaust gas containing CO, HC (hydrocarbon) and NOx (nitrogen oxide), which is purified by a catalyst before being released to the atmosphere. Yes. An example thereof is described in Patent Document 1, and in this method, when purifying exhaust gas in an oxygen-excess atmosphere, a porous carrier carries one of oxides of barium, calcium, and magnesium and platinum. In addition, the exhaust gas is brought into contact with a catalyst in which the supported amount of the oxide is 0.05 to 10 mol with respect to 1 liter of the support. As a method for producing the catalyst, Patent Document 1 discloses a method in which a porous carrier carrying platinum is immersed in the above alkaline earth metal acetate aqueous solution to absorb water, and then dried and fired. Is described.
Japanese Patent No. 3328318

  As the carrier, a porous body having a large number of pores is usually used in order to increase the surface area, and the alkaline earth metal acetate aqueous solution is absorbed into the porous body and penetrates. As a result, the alkaline earth metal is widely dispersed. When this is dried, water evaporation occurs on the surface of the carrier, but the capillary pressure generated at the open end of the pores constituting the carrier is strong, so that the aqueous solution is caused by the capillary pressure during the drying process. Move to the surface side. For this reason, the concentration of acetate increases on the surface side of the carrier, and water evaporates in this state, so that the particle size of the deposited NOx storage material becomes larger on the surface side of the carrier.

Since the exhaust gas brought into contact with the catalyst contains sulfur oxides such as SO _X (sox), it adheres not only to catalytically active materials such as platinum but also to the surface of the NOx storage material, Sulfur poisoning may occur due to the alkaline earth metal sulfate compound that forms the NOx storage material. If the particle size of the sulfuric acid compound is small, it is relatively unstable and can be decomposed in a high temperature reducing atmosphere to regenerate sulfur poisoning. However, if the particle size of the NOx occlusion material is large, the particle size of the sulfate compound produced by adhering to it tends to be large, and the sulfate compound thus enlarged is stable. In particular, it is almost impossible to regenerate poisoning at low temperatures. As a result, the NOx storage function and the exhaust gas purification function may be impaired.

  The present invention has been made by paying attention to the above technical problem, and is a catalyst that can support NOx occlusion material in a highly dispersed state in a finely dispersed state and prevent or suppress the coarsening of the particle size. The object is to provide a manufacturing method.

  In order to achieve the above object, the present invention provides a catalyst having a NOx occlusion material in which a compound of one or more elements of alkali metal, alkaline earth and rare earth is supported on a carrier as an NOx occlusion material In the method, the compound of the element used as the NOx storage material is converted into a complex ion having a reverse potential to the carrier with an organic acid, the complex ion is adsorbed on the carrier, and the carrier is dried and calcined. It is a method. The organic acid may be any one of succinic acid, malic acid, tartaric acid and citric acid as described in claim 2.

  According to the present invention, a compound of one or more elements of alkali metal, alkaline earth, and rare earth is converted into a complex ion in a predetermined solution by the organic acid. Therefore, in this state, since the compound of the element is not solidified, it is not bonded and coarsened. Since the complex ion has a potential opposite to that of the carrier, when the solution is absorbed by the porous carrier, the complex ion is widely dispersed and adsorbed on the carrier due to electrical interaction between the two. . When this is dried and fired, even though moisture moves from the inside of the pores of the carrier to the surface side, the compound of the element is strongly adsorbed by electrical interaction with the carrier. The movement is prevented or suppressed, and the ions are dried as they are and supported on the carrier. As a result, since the highly dispersed state of the NOx occlusion material is maintained, concentration during the drying process and coarsening associated therewith are suppressed or prevented. That is, the NOx occlusion material is supported in a highly dispersed state in the form of fine particles, and a catalyst excellent in sulfur poisoning regeneration can be obtained.

  The present invention is a method for producing a catalyst in which a NOx storage material is supported on a carrier. The oxide serving as the carrier is a heat-resistant or refractory porous body having pores of a predetermined diameter, and the pore diameter is mesopores that enter a so-called meso region of about 10 to 100 nm. preferable. Specifically, zirconia, alumina, silica alumina, lanthanum yttria zirconia composite oxide, yttria zirconia composite oxide, lanthanum zirconia composite oxide, ceria zirconia composite oxide, and the like can be employed. As an example, the carrier particles made of the oxide are porous structures obtained by further aggregating the aggregated secondary particles of the primary particles generated by hydrolysis.

  On the other hand, the NOx occlusion material supported on the carrier is a finer particle than the carrier particle, and is, for example, a so-called ultrafine particle of about several nm. Specifically, fine particles of oxides of alkali metals, alkaline earth metals, rare earths and the like having a NOx occlusion function.

  In the method of the present invention, a complex ion of a compound containing an element that becomes a NOx storage material is synthesized in advance. The surface potential is adjusted to a potential opposite to that of the carrier on which it is to be supported or its precursor. First, a method for synthesizing complex ions of the NOx occlusion material will be described. An aqueous solution is prepared by dissolving an alkali metal, alkaline earth metal or rare earth metal salt used as the NOx occlusion material in water. The aqueous solution is, for example, a barium acetate aqueous solution or a calcium nitrate aqueous solution. An organic acid is added to the aqueous solution to form complex ions. Examples of organic acids that can be used to form complex ions in the present invention include polyvalent organic acids such as succinic acid, malic acid, tartaric acid, and citric acid.

  Since the stability of the complex ions formed in this manner varies depending on the pH of the aqueous solution, it is adjusted to a pH that is as stable as possible. A method for determining the stability of the complex ions will be described. FIG. 1 is a curve showing the results of measuring the change in pH of citric acid dropped into a calcium nitrate aqueous solution and a barium acetate aqueous solution. Specifically, the alkaline earth metal ion concentration is 0.0827 mol, citric acid 0.248 mol, and the solution volume is 50 ml. The diluted solution is dropped, and the pH in the process is sequentially measured and expressed in a diagram.

In the low pH region (about 4.2 or less for barium acetate and about 3.5 or less for calcium nitrate), complex ions are not dissolved but precipitate and are separated from the liquid phase. The remaining acid lowers the pH. In the intermediate pH range (about 4.2 to 5.4 for barium acetate and about 3.5 to 4.8 for calcium nitrate), the buffering action of complex ions, that is, the higher the pH, the greater the degree of divergence. As a result, the slope of the pH titration curve is reduced. Furthermore, in the high pH range (about 5.4 or more for barium acetate and about 4.8 or more for calcium nitrate), complex ions are decomposed by adding a base, and alkaline earth ( ²⁺ ) ions of strong bases are added. Begins to liberate, so the pH rises abruptly. That is, the slope of the pH titration curve increases. Since such a pH titration curve is obtained for the salt of each element used as the NOx storage material, the pH of the aqueous solution to which the organic acid is added may be adjusted with the region having a small slope as the stable region of complex ions. Note that the stable region of complex ions is somewhat narrower than the complex ion formation region (that is, the pH range in which complex ions can be formed).

  The surface potential of the complex ion and the carrier obtained as described above will be described. The complex ion formed by the organic acid described above is negatively charged. The surface potential (zeta potential) of the carrier made of the complex oxide is positive. An example thereof is shown in FIG. 2, and in a region of pH around 4.0 to 5.5 where citric acid is added to an aqueous barium acetate solution to form a stable complex ion, a carrier such as alumina is positively charged. Such a tendency is the same for the complex ions of other carriers and NOx occlusion materials. Therefore, in the present invention, complex ions having a potential opposite to that of the carrier are formed by the organic acid.

  Since the complex ions stabilized by adjusting the pH exist as ions in the aqueous solution, elemental compounds (Ba citrate, Ca citrate, etc.) that become NOx storage materials precipitate. There is no. Therefore, it is not necessary to carry out the supporting operation on the carrier immediately after the synthesis of the elemental compound that becomes the NOx storage material, and the handling becomes easy.

  Next, a method for supporting the NOx occlusion material on the carrier will be described. An appropriate method such as a so-called water absorption method or impregnation method known in the art can be employed. Specifically, a porous carrier prepared in advance is immersed in the above complex ion aqueous solution, and then pulled up, and the excess aqueous solution is blown off to sufficiently absorb or impregnate the complex ion aqueous solution. The carrier may be either a monolith type or a pellet type.

  In the case of carrying in this way, as described above, the complex ions are negatively charged, and the carrier is charged positively opposite to this, so that the NOx occlusion material on the carrier due to the electrical interaction between them. Are adsorbed. This is due to an electrical action, and the complex ions are adsorbed at positive points on the carrier, so that the complex ions are adsorbed on the carrier in a widely dispersed state.

  The carrier on which the NOx occlusion material is adsorbed in the form of complex ions is then used as a catalyst after being dried and calcined. In the drying process, moisture evaporates from the surface of the carrier, whereas the carrier has a porous structure. Therefore, capillary pressure is generated on the surface side of the carrier, and moisture inside gradually moves to the surface side. However, since the complex ions of the NOx storage material contained in the aqueous solution are adsorbed on the carrier due to the electrical interaction with the carrier, even if the water moves toward the surface of the carrier, it is complex. Ion migration is blocked or suppressed. Therefore, coarsening due to aggregation or concentration of NOx occlusion material is prevented or suppressed, and a highly dispersed state with a fine particle size is maintained.

  The NOx storage material is supported on a carrier together with a catalytically active material made of a noble metal such as platinum. Alternatively, the catalytically active material may be supported first, and then the NOx storage material may be supported. The reverse order may be used. When the catalytically active substance is first supported, the NOx occlusion material or precursor thereof in the method of the present invention is fine enough to be dispersed in the form of complex ions and referred to as ultrafine particles. The effect on the exposure of much larger noble metal particles is small.

  The NOx storage material is adsorbed on the support in the form of complex ions, dried in that state, loses moisture, and is fixed on the support in the form of solids (ie particles), and becomes an oxide. That is, it is carried. Since the movement of complex ions or particles is prevented or suppressed in the process as described above, the supported NOx storage material maintains a highly dispersed state close to the dispersed state in the complex ion state. For this reason, coarsening due to aggregation of particles is prevented or suppressed. Therefore, the particle size of the NOx occlusion material in the obtained catalyst is small and dispersed in a highly dispersed state, and the supported amount (the supported amount per unit amount of the catalyst) can be increased more than ever. Further, the NOx occlusion material rarely inhibits exposure of catalytically active substance particles such as noble metals, and the activity is not impaired.

  Thus, according to the method of the present invention, the amount of the NOx occlusion material supported can be increased, so that an exhaust gas purification catalyst having a large NOx occlusion amount can be obtained. Further, since the NOx occlusion material can be maintained in an ultrafine particle state, even if it is subjected to sulfur poisoning, the generated sulfate compound is unstable and can be easily regenerated from sulfur poisoning. A catalyst with high purification performance can be obtained.

  Next, examples and comparative examples performed for confirming the effects of the present invention will be described.

〔Example〕
Add 3 times molar aqueous diammonium hydrogen citrate solution to barium acetate aqueous solution, add a small amount of ammonia water to adjust pH to 4.0-5.5, and negatively charged barium citrate complex ion. Form. In this pH region, it was identified from the pH titration curve that it does not precipitate as barium citrate and exists as an ion. A monolithic carrier prepared by a general method known in the art and carrying platinum was immersed in this liquid. The complex ions were adsorbed relatively uniformly on the monolith support. After drying at 250 ° C., the catalyst was calcined in air at 500 ° C. for 1 hour to obtain a catalyst. It was confirmed that the loading efficiency at this time was almost 100% from the weight change, assuming that the complex ion after calcination is changed to carbonate, so that carbonate is assumed. The supported amount of barium was 0.2 mol (0.2 mol / L) with respect to 1 liter of the catalyst, and the supported amount of platinum (Pt) was 1.5 g / L.

[Comparative Example 1]
Barium was supported by a conventional water absorption support method. That is, a carrier previously supported with platinum is immersed in an aqueous barium acetate solution having a predetermined concentration, and the aqueous barium acetate solution is absorbed by the carrier, and then the excess aqueous solution is blown off under the same conditions as in the above examples. The catalyst was obtained by drying and calcining. The supported amount of barium was 0.1 mol / L, and the supported amount of platinum (Pt) was 3 g / L. This is a conventional standard NOx catalyst for diesel engines, in which the supported amount of barium is reduced and the supported amount of platinum is increased in order to increase the low temperature activity.

[Comparative Example 2]
After synthesizing barium carbonate, it was supported by mixing an aqueous barium carbonate solution with a carrier charged to the opposite potential. This is a so-called fine particle adsorption method. First, a carrier on which platinum is previously supported (alumina carrier) and a pyromellitic acid saturated solution are mixed to invert the surface potential (zeta potential) of the alumina carrier to minus. On the other hand, a barium acetate aqueous solution having a predetermined concentration and an aqueous ammonium hydrogen carbonate solution are mixed to adjust the pH to 6.5, thereby generating barium carbonate particles having a positive surface potential. Thereafter, these solutions are mixed and stirred for a predetermined time to adsorb barium carbonate particles on the carrier. Thereafter, the carrier on which the barium carbonate particles were adsorbed was filtered off, and dried and calcined in the same manner as in the above example to obtain a catalyst. The amount of barium supported was 0.2 mol / L and the amount of platinum (Pt) supported was 1.5 g / L, as in the examples.

[Comparative Example 3]
Barium was supported by a conventional water absorption support method. The carrying amount was only different from that of Comparative Example 1 described above, and the other conditions were the same as those of Comparative Example 1. That is, the supported amount of barium was 0.2 mol / L, and the supported amount of platinum (Pt) was 1.5 g / L.

[Evaluation]
The catalysts obtained in Examples and the catalysts obtained in Comparative Examples 1 and 2 were evaluated for poisoning by SO _X (sock) and regeneration from poisoning. As exhaust gas of the internal combustion engine, exhaust gas at the time of lean combustion (lean) and exhaust gas at the time of rich or stoichiometric or an air / fuel ratio smaller than that were flowed. The space velocity SV = 50000 h ⁻¹ , the lean operation was continued for 1 minute, and then the operation in which the rich operation was performed for 30 seconds was continued for a predetermined time. The composition of the exhaust gas during the lean operation is NOx: 250 ppm, O ₂ : 6%, H ₂ O: 8%, CO ₂ : 10%. The composition of the exhaust gas during the rich operation is the same as that of the lean operation except for O ₂ : 0%, CO: 6000 ppm.
The sulfur poisoning conditions were a temperature of 250 to 550 ° C. and a passing sulfur amount of 6 g (6 g / L) with respect to 1 liter of catalyst. The sulfur regeneration conditions were such that an exhaust gas corresponding to a temperature of 650 ° C. and an air-fuel ratio (A / F) of “14” was circulated for 10 minutes. The obtained results are shown in Table 1.

  Table 1 shows the NOx purification rate at repeated sox poisoning regeneration. With the catalyst obtained in the example of the present invention, the NOx purification rate at each temperature range when new and after repeated sox poisoning regeneration. It was found that the NOx occlusion material was not easily poisoned by sulfur, or that it was easy to regenerate from sulfur poisoning. This is thought to be due to the refinement of barium, which is a NOx storage material, and the degree to which precious metals such as platinum and rhodium are exposed without being covered by the NOx storage material is high. It seems to be due to.

  On the other hand, in the catalyst according to Comparative Example 1 in the conventional water-absorbing support method, it is recognized that the NOx purification rate after repeated sox poisoning regeneration decreases, and this tendency becomes more prominent as the temperature increases, and at 550 ° C. It was found that it decreased to about 1/8, was easily poisoned by sulfur, and its regeneration was difficult. This seems to be due to the fact that the diameter is increased although it is a NOx storage material.

  Further, in the catalyst of Comparative Example 2 in the so-called fine particle adsorption supporting method, it is recognized that the NOx purification rate after repeated sox poisoning regeneration decreases as in the catalyst of Comparative Example 1, and the tendency is higher as the temperature is higher. It was noticeable that it was easily poisoned by sulfur and its regeneration was difficult. In Comparative Example 2, the barium carbonate particles and the carrier are adjusted to the reverse potential and supported by the electrical interaction between them, but after synthesizing the barium carbonate particles that are precursors of the NOx storage material, This is presumably due to the fact that the barium carbonate particles are brought into contact with each other and bonded to each other while being supported on the carrier and are coarsened and supported on the carrier in this state, and as a result, the NOx occlusion material is enlarged.

Next, the NOx purification rate at low temperatures after the thermal endurance test was evaluated for the catalyst according to the embodiment of the present invention and the catalysts according to Comparative Examples 1 and 3 described above. Exhaust gas at the time of lean combustion (lean) and exhaust gas at the time of rich with a small air-fuel ratio were flowed as exhaust gas of the internal combustion engine. The space velocity SV = 20000h ⁻¹ , the lean operation was continued for 20 seconds, and then the operation in which the rich operation (so-called rich spike) was performed for 5 seconds was continued for a predetermined time. The composition of the exhaust gas during lean operation is NOx: 50 ppm, O ₂ : 10%, H ₂ O: 5%, CO ₂ : 6%. The composition of the exhaust gas during the rich operation is the same as that of the lean operation except for O ₂ : 0%, hydrocarbon in terms of carbon: 3000 ppmC. Further, the heat durability was 750 ° C. for 2 hours, the lean air-fuel ratio (A / F) was “40”, and the rich air-fuel ratio (A / F) was “14”. The obtained results are shown in Table 2.

  The catalyst prepared by the method of the present invention is the same as the catalyst of Comparative Example 1 in which the amount of NOx occlusion material is decreased and the amount of platinum is increased to support the water absorption of the NOx occlusion material in order to improve low temperature activity. The degree of NOx purification was shown. In contrast, in Comparative Example 3 in which the supported amount of barium and platinum was the same as that of the catalyst according to the method of the present invention and the water absorption supporting method was adopted as the supporting method, the NOx purification rate at low temperatures of 250 ° C. and 200 ° C. Decreased. That is, according to the method of the present invention, it was recognized that the low-temperature activity was improved even if the amount of the catalytically active substance composed of a noble metal such as platinum was reduced.

It is a curve which shows the result of having dripped citric acid to calcium nitrate aqueous solution and barium acetate aqueous solution, and measuring the change of the pH. It is a diagram which shows the relationship of the surface potential of the support | carrier and complex ion in the method of this invention.

Claims (2)

In a method for producing a catalyst having a NOx occlusion material in which a compound of one or more elements of alkali metal, alkaline earth and rare earth is supported on a carrier as an NOx occlusion material,
The NOx storage material is made of NOx, characterized in that the elemental compound is converted into a complex ion having a reverse potential to the carrier by an organic acid, the complex ion is adsorbed on the carrier, and the carrier is dried and calcined. A method for producing a catalyst having an occlusion material.   The method for producing a catalyst having a NOx storage material according to claim 1, wherein the organic acid is any one of succinic acid, malic acid, tartaric acid, and citric acid.

Images