JP2005199122A - Exhaust gas purifying catalyst, its manufacturing method and exhaust gas purifying catalyst device for vehicle - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust gas purifying catalyst capable of developing excellent purifying characteristics by baking a composite oxide of noble metals same as a conventional one at a low temperature and preventing the lowering in a Pd dispersion degree due to the advance of crystallization caused by baking. <P>SOLUTION: The exhaust gas purifying catalyst is a Pd based composite oxide containing at least one kind selected from alkaline earth metals. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an exhaust gas purifying catalyst, a method for producing the same, and an exhaust gas purifying catalyst device for vehicles, and in particular, nitrogen oxides (NOx), hydrocarbons (HC) and one in exhaust gas discharged from an internal combustion engine such as an automobile. The present invention relates to a technology for producing an exhaust gas purification catalyst capable of efficiently purifying and reducing carbon oxide (CO) simultaneously.

It is known that noble metal elements (Pt, Rh, Pd, Ir) exhibit high performance for purification of exhaust gas (for example, CO, HC, NO, NO _2, etc.). For this reason, it is suitable to use the said noble metal element for an exhaust gas purification catalyst. Usually, these noble metals are used by being mixed or supported on Al ₂ O ₃ as a high specific surface area support together with additives such as La, Ce and Nd. On the other hand, it is known that a noble metal can be combined with various elements by converting it into a complex oxide, whereby various properties can be obtained compared to the noble metal alone and the exhaust gas purification performance can be improved. Furthermore, it is known that an exhaust gas purification catalyst having excellent heat resistance can be obtained in a solid solution state or mixed state of a Pd-based composite oxide and a transition metal-based composite oxide.

  In such an exhaust gas purification catalyst, a state in which a Pd-based composite oxide consisting of at least one selected from rare earth metals or alkaline earth metals and a composite oxide consisting of at least one transition metal are dissolved or mixed. Thus, a heat-resistant catalyst used as a coexisting composite oxide has been proposed (see Patent Document 1). In such a conventional technique, the heat resistance can be improved based on stabilization of fine Pd particles by using a Pd-based composite oxide as the exhaust gas purification catalyst. Further, the heat resistance can be improved based on prevention of sintering of the Pd-based composite oxide by mixing the Pd-based composite oxide and the transition metal-based composite oxide.

JP-A-10-277393 (Claims)

  However, in order to produce a Pd-based composite oxide in the above prior art, it is necessary to calcinate at 1000 ° C. or higher. For this reason, crystallization progresses, Pd dispersion degree falls, and there exists a possibility that the outstanding purification characteristic may not be acquired. Therefore, development of an exhaust gas purification catalyst capable of preventing the decrease in the Pd dispersion degree due to the progress of crystallization and exhibiting excellent purification characteristics even when a complex oxide having the same amount of noble metal as before is calcined at a lower temperature. Technology was requested.

  The present invention has been made in view of the above circumstances, and a composite oxide having the same amount of noble metal as in the prior art is fired at a lower temperature, thereby preventing a decrease in Pd dispersion due to the progress of crystallization, and is excellent. An object of the present invention is to provide an exhaust gas purification catalyst capable of exhibiting purification characteristics, a method for producing the same, and a vehicle exhaust gas purification catalyst device.

The present inventors diligently and researched about an exhaust gas purification catalyst capable of exhibiting excellent purification characteristics in accordance with the above object and a method for producing the same. As a result, by using a Pd-based composite oxide containing an alkaline earth metal for the exhaust gas purification catalyst, even if the amount of the noble metal is the same as the conventional amount, Pd is present more on the surface and the Pd dispersibility is greatly improved. It has been found that the growth of particles can be suppressed and the purification properties can be improved. Further, the Pd-based composite oxide is A ₂ PdO ₃ (A: alkaline earth metal), and a plurality of Pd-based composite oxides having a composition of A ₂ PdO ₃ are allowed to coexist so that different Pd-based composite oxides are present. The knowledge that the initial Pd dispersibility and heat resistance can be further improved by securing an interatomic distance between the composite oxides to a certain extent by using an element other than Pd as a blocking material among the objects. Obtained. Further, in the heat treatment of the Pd-based composite oxide, even when the Pd-based composite oxide is fired at a lower temperature than in the past, the dispersion degree of Pd in the Pd-based composite oxide is kept high, and the purification characteristics are maintained. The knowledge that can be improved. The present invention has been made based on these findings.

  That is, the exhaust gas purification catalyst of the present invention is characterized in that it is a Pd-based composite oxide containing at least one selected from alkaline earth metals. Here, the Pd-based composite oxide is, for example, a Pd-based alkaline earth metal composite oxide, and examples of A include Sr, Ba, and Ca.

In such an exhaust gas purification catalyst, the Pd-based composite oxide is preferably A ₂ PdO ₃ (A: alkaline earth metal).

  Further, it is desirable that such an exhaust gas purification catalyst forms a complex by coexisting an organic acid when mixing a metal salt as a raw material as a solution. Specifically, in the exhaust gas purification catalyst, the Pd-based composite oxide is a compound group (an OH group or an SH group having 2 to 20 carbon atoms, a carbon number 2 or 3 dicarboxylic acid, and carbon. It is desirable that it is produced through a step of adding at least one selected from (monocarboxylic acids of several 1 to 20) to the nitrate aqueous solution of the constituent elements.

  In addition, as a specific example of a C2-C20 carboxylic acid which has OH group or SH group, the compound which substituted the oxygen atom of OH group of oxycarboxylic acid and oxycarboxylic acid by the sulfur atom is mentioned. Carbon number of these carboxylic acids is 2-20 from a soluble viewpoint to water, 2-12 are preferable, 2-8 are more preferable, and 2-6 are more preferable. Moreover, carbon number of monocarboxylic acid is 1-20 from a soluble viewpoint to water, 1-12 are preferable, 1-8 are more preferable, and 1-6 are more preferable. Further, specific examples of the carboxylic acid having 2 to 20 carbon atoms having an OH group or an SH group include glycolic acid, mercaptosuccinic acid, thioglycolic acid, lactic acid, β-hydroxypropionic acid, malic acid, tartaric acid, citric acid, Examples include isocitric acid, allocic acid, gluconic acid, glyoxylic acid, glyceric acid, mandelic acid, tropic acid, benzylic acid, and salicylic acid. Specific examples of monocarboxylic acids include formic acid, acetic acid, propionic acid, butyric acid, isobutyric acid, valeric acid, isovaleric acid, hexanoic acid, heptanoic acid, 2-methylhexanoic acid, octanoic acid, 2-ethylhexanoic acid, and nonane. Examples include acid, decanoic acid, and lauric acid. Among these, acetic acid, oxalic acid, malonic acid, glycolic acid, lactic acid, malic acid, tartaric acid, glyoxylic acid, citric acid and gluconic acid are preferable, and oxalic acid, malonic acid, glycolic acid, lactic acid, malic acid, tartaric acid, glyoxyl More preferred are acids, citric acid and gluconic acid.

  Furthermore, in such an exhaust gas purification catalyst, the nitrate aqueous solution was evaporated to dryness to produce a carboxylic acid complex polymer, and a calcination step of firing the carboxylic acid complex polymer. Is desirable. In addition, as an example of the said baking process, an organic acid and salt root are removed by temporary baking in the air, a temporary baking body is produced, and after the temporary baking body is wrought, this is performed at 750 ° C. for 10 hours in the air. What performs baking is mentioned.

  Next, the method for producing an exhaust gas purification catalyst of the present invention is a method for producing the above exhaust gas purification catalyst suitably, and the exhaust gas purification of a Pd-based complex oxide containing at least one selected from alkaline earth metals In producing the catalyst, at least selected from a compound group (a carboxylic acid having 2 to 20 carbon atoms having an OH group or an SH group, a dicarboxylic acid having 2 or 3 carbon atoms, and a monocarboxylic acid having 1 to 20 carbon atoms). It includes a step of adding one kind to a nitrate aqueous solution of a constituent element.

  Such a method for producing an exhaust gas purification catalyst preferably includes a step of evaporating and drying the aqueous nitrate solution to produce a carboxylic acid complex polymer, and a firing step of firing the carboxylic acid complex polymer. Moreover, it is more desirable that the firing temperature in the firing step is 900 ° C. or lower.

  Although the exhaust gas purification catalyst and the production method thereof as described above are the outline of the present invention, the present inventors have intensively studied the specific uses of these inventions described above, and the exhaust gas purification catalyst of the present invention is The inventors have obtained the knowledge that it is particularly suitable for use in automobiles among internal combustion engines, and have completed the following invention.

  That is, the vehicle exhaust gas purification catalyst device of the present invention includes a Pd-based composite oxide containing at least one selected from alkaline earth metals, and the Pd-based composite oxide purifies exhaust gas discharged from the vehicle. It is characterized by doing.

According to the present invention, in an exhaust gas purification catalyst comprising a Pd-based composite oxide, by containing an alkaline earth metal in the Pd-based composite oxide, a larger amount of Pd is present on the surface even with the same amount of metal as in the past. Thus, Pd dispersibility can be greatly improved, particle growth between Pd can be suppressed, and purification characteristics can be improved. That is, as shown in FIG. 1, in the Pd-based composite oxide of the present invention, Pd ²⁺ is suitably dispersed on the support. In the figure, A is an alkaline earth metal. On the other hand, FIG. 2 shows PdO as a conventional exhaust gas purification catalyst, and there are places where Pd ²⁺ is suitably dispersed, but some of Pd exists in a metal state with low reactivity with exhaust gas. There are also places that do. Therefore, the present invention is an exhaust gas purification capable of efficiently and simultaneously purifying and reducing nitrogen oxides (NOx), hydrocarbons (HC) and carbon monoxide (CO) in exhaust gas discharged from an internal combustion engine such as an automobile. It is promising in that it can provide catalyst manufacturing technology. Examples of nitrogen oxides (NO _X ) include NO and NO ₂ .

Hereinafter, the present invention will be described in more detail with reference to examples.
<Production Example 1>
A mixed metal nitrate aqueous solution was prepared by dissolving 2.96 g (0.014 mol) of strontium nitrate, 1.86 g (0.007 mol) of palladium nitrate and 3.75 g (0.028 mol) of malic acid in 100 ml of ion-exchanged water. Produced. Next, the mixed metal nitrate aqueous solution was prepared and evaporated to dryness at 250 ° C. while stirring with a hot plate stirrer. Thereafter, this was transferred to an alumina crucible, heated to 350 ° C. at 2.5 ° C./min in a muffle furnace, and heat-treated at 350 ° C. for 3 hours. Thus, a temporarily fired body from which nitrate radicals were removed was produced.

The calcined body was pulverized in a mortar for 15 minutes, then placed in an alumina crucible again, heated to 750 ° C. at 5 ° C./min in a muffle furnace, and held at 750 ° C. for 10 hours for main firing. Next, catalyst powder, water, pulverizing balls, SiO ₂ sol and alumina, which are Pd composite oxides, were placed in a container and pulverized and mixed for 14 hours in a ball mill to obtain a slurry. Thereafter, the slurry was supported on the honeycomb by a predetermined weight and then held at 750 ° C. for 3 hours. As described above, an exhaust gas purification catalyst of Sr ₂ PdO ₃ was obtained.

<Production Example 2>
Instead of strontium nitrate used in Production Example 1, barium nitrate was used to obtain an exhaust gas purification catalyst of Ba ₂ PdO ₃ . Other conditions were the same as in Production Example 1.

<Production Example 3>
Palladium nitrate and malic acid were dissolved in ion-exchanged water, and then PdO was produced in the same manner as in Production Example 1. Next, a catalyst was produced from this PdO and alumina in the same manner as in Production Example 1.

<Production Example 4>
A mixed aqueous solution of palladium nitrate and strontium nitrate was neutralized with ammonium carbonate and concentrated to obtain a paste-like mixture. Then, after thermal decomposition at 300 ° C., and calcined for 3 hours at 1000 ° _C., to obtain a Sr 2 PdO ₃ powder. After the obtained powder was pulverized, it was mixed with alumina in the same manner as in Production Example 1, and a predetermined amount was supported on the honeycomb to obtain an exhaust gas purification catalyst of Sr ₂ PdO ₃ .

Each exhaust gas purification catalyst obtained as described above was subjected to an activity evaluation in the initial stage and after the endurance treatment. In the initial activity evaluation, model exhaust gases having air-fuel ratios of 14.3 and 14.9 are repeatedly passed through each catalyst in a cycle of 0.5 seconds (cycle: 1 Hz), and the flow rate per unit time and unit volume is 50000 h ^−1. Correspondingly, the reaction was carried out at a reaction temperature of 30 to 400 ° C. The durability treatment was performed for 20 hours at a gas temperature of 900 ° C. using a model exhaust gas having an air-fuel ratio of 14.6. Under such conditions, the evaluation after the durability treatment was performed. Table 1 shows the temperature rise test conditions, and Tables 2 and 3 show the results of each activity evaluation. That is, Table 2 shows the 50% purification temperature of CO, HC, and NO and the purification rate at 400 ° C. in the temperature increase test of the initial catalyst. Table 3 shows the 50% purification temperature of CO, HC and NO and the purification rate at 400 ° C. in the temperature rise test of the catalyst after the durability treatment.

  According to Tables 2 and 3, Production Examples 1 and 2 within the scope of the present invention have a relatively low 50% purification temperature and a high 400 ° C. purification rate both in the initial stage and after the endurance treatment. You can see that. On the other hand, it can be seen that Production Examples 3 and 4 outside the scope of the present invention have a relatively high 50% purification temperature and a low 400 ° C. purification rate both in the initial stage and after the endurance treatment. .

  Next, FIGS. 3 to 5 show the relationship between the CO, HC and NO purification rates and the temperatures for the exhaust gas purification catalysts of Production Examples 1 to 4. FIG. In each of these figures, (a) shows the temperature rise characteristic for the initial catalyst, and (b) shows the temperature rise characteristic for the catalyst after the durability treatment. As is apparent from these drawings, Production Examples 1 and 2 within the scope of the present invention generally exhibit excellent purification characteristics as compared with Production Examples 3 and 4 outside the scope of the present invention from around 200 ° C. I understand that.

As described above, it has been found that Production Examples 1 and 2 within the scope of the present invention show excellent purification characteristics with respect to Production Examples 3 and 4 outside the scope of the present invention. In addition, the Pd dispersion degree was evaluated. Specifically, each of the exhaust gas purifying catalysts of Production Examples 1 to 4 that were prototyped was measured at a gas temperature of 50 ° C. by the CO adsorption method. That is, in measuring the amount of adsorption of CO, the CO pulse method is used, and as a pretreatment, each exhaust gas purification catalyst is exposed to O ₂ at 400 ° C. for 15 minutes and H ₂ at 400 ° C. for 15 minutes, and the measurement temperature is 50 ° C. It was. The Pd amount was 0.75 g / L. The result is shown in FIG.

As is apparent from FIG. 6, Production Examples 1 and 2 within the scope of the present invention have higher Pd dispersion than Production Examples 3 and 4 outside the scope of the present invention, and are generally values of 10% or more. It can be seen that In the figure, the dispersity of Production Example 3 is very low because the exhaust gas purification catalyst is made of a mixture of PdO and alumina, and there is a portion where Pd does not exist at all locally. In addition, the dispersion of Production Example 4 in the figure is lower than the dispersion of Production Examples 1 and 2, because the mixed aqueous solution of palladium nitrate and lanthanum nitrate was neutralized with ammonium carbonate during the production process and concentrated. This is because a paste-like mixture was obtained and then Sr ₂ PdO ₃ powder was obtained, and the process of adding malic acid or the like, which is a preferred production process of the present invention, to the nitrate aqueous solution was not performed.

  The exhaust gas purifying catalyst of the present invention is an internal combustion engine such as an automobile that is required to efficiently purify and reduce nitrogen oxide (NOx), hydrocarbon (HC) and carbon monoxide (CO) in exhaust gas at the same time in recent years. Applicable to institutions.

It is a schematic diagram which shows the Pd dispersion state of Pd type complex oxide which comprises the exhaust gas purification catalyst of this invention. It is a schematic diagram which shows the Pd dispersion state of the Pd type oxide which comprises the conventional exhaust gas purification catalyst. It is a graph which shows the relationship between the CO purification rate and temperature about each exhaust gas purification catalyst of manufacture examples 1-4, (a) shows the temperature rising characteristic about an initial stage catalyst, (b) The temperature rise characteristic about the catalyst after an endurance process is shown. It is a graph which shows the relationship between the purification rate of HC and temperature about each exhaust gas purification catalyst of manufacture examples 1-4, (a) shows the temperature rising characteristic about an initial stage catalyst, (b) is The temperature rise characteristic about the catalyst after an endurance process is shown. It is a graph which shows the relationship between the NO purification rate and temperature about each exhaust gas purification catalyst of manufacture examples 1-4, (a) shows the temperature rising characteristic about an initial stage catalyst, (b) is The temperature rise characteristic about the catalyst after an endurance process is shown. It is a graph which shows Pd dispersion degree about each exhaust gas purification catalyst of manufacture examples 1-4.

Claims (8)

  An exhaust gas purifying catalyst, which is a Pd-based composite oxide containing at least one selected from alkaline earth metals. _2. The exhaust gas purification catalyst according to claim 1, wherein the Pd-based composite oxide is A ₂ PdO ₃ (A: alkaline earth metal).   The Pd-based composite oxide is selected from a compound group (a carboxylic acid having 2 to 20 carbon atoms, an OH group or an SH group, a dicarboxylic acid having 2 or 3 carbon atoms, and a monocarboxylic acid having 1 to 20 carbon atoms). The exhaust gas purifying catalyst according to claim 1 or 2, wherein the exhaust gas purifying catalyst is manufactured through a step of adding at least one kind to a nitrate aqueous solution of a constituent element.   The exhaust gas according to claim 3, wherein the exhaust gas is produced through a step of evaporating and drying the aqueous nitrate solution to produce a carboxylic acid complex polymer, and a firing step of firing the carboxylic acid complex polymer. Purification catalyst. A method for producing an exhaust gas purification catalyst of a Pd-based composite oxide containing at least one selected from alkaline earth metals,
At least one selected from the group of compounds (a carboxylic acid having 2 to 20 carbon atoms having an OH group or an SH group, a dicarboxylic acid having 2 or 3 carbon atoms, and a monocarboxylic acid having 1 to 20 carbon atoms) is used as a constituent element. A method for producing an exhaust gas purification catalyst comprising a step of adding to an aqueous nitrate solution.   The exhaust gas purifying catalyst according to claim 5, comprising a step of evaporating and drying the nitrate aqueous solution to produce a carboxylic acid complex polymer, and a firing step of firing the carboxylic acid complex polymer. Production method.   The method for producing an exhaust gas purification catalyst according to claim 6, wherein a firing temperature in the firing step is 900 ° C. or less.   An exhaust gas purification catalyst device for vehicles, comprising a Pd-based composite oxide containing at least one selected from alkaline earth metals, wherein the Pd-based composite oxide purifies exhaust gas discharged from a vehicle.

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