JPH0586259B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to an exhaust gas purifying catalyst comprising palladium contained in a defective perovskite composite oxide. [Prior art] Three types of technology that can simultaneously purify hydrocarbons (HC), carbon monoxide (CO), and nitride oxides (NO _x ), which are harmful substances contained in exhaust gas emitted from internal combustion engines such as automobiles. As a base catalyst, a noble metal supported on an alumina carrier has been put into practical use. The noble metal that is the catalyst component is platinum (Pt).
Combinations of rhodium (Rh), palladium (Pd)-rhodium (Rh), platinum-palladium-rhodium, etc. are used. However, in any of the catalysts, Rh, which is a rare and expensive resource, is required to reduce NO _x . Further, although the three-way catalyst has high activity, there is a problem that if it is used for a long time at a high temperature of 900° C. or higher, it deteriorates due to sintering of the noble metal and a decrease in the specific surface area of the alumina support. On the other hand, composite oxides with a perovskite structure made of a combination of rare earth elements and transition metals have excellent thermal stability, and have therefore been viewed as promising as catalysts for purifying automobile exhaust gas, and research has been progressing. Among them, La _1-x Sr _x CoO ₃ has become comparable to Pt catalysts in terms of HC and CO oxidation activity, but it has the drawback of very low NO _x purification ability. It is not yet practical as a catalyst for automobiles. Therefore, as a measure to improve the NO _x purification ability of complex oxides with a perovskite structure,
It is known that a noble metal is dissolved in a composite oxide having a perovskite structure, or an oxide of a rare earth element such as cerium oxide is added to support the noble metal (for example, JP-A No. 1989-1999-1).
No. 83295, JP-A-50-78567, JP-A-58-156349
No. 162948 (Japanese Patent Publication No. 162948). However, in the case of forming a support in the former precious metal, the perovskite composite oxide itself is fired at a high temperature of nearly 1000°C when the noble metal ion is substituted into the _B site of the perovskite structure with the general formula ABO 3 to form a solid solution. The activity decreases as the specific surface area of the compound decreases. Furthermore, in order to obtain a high NO _x purification ability, it is necessary to use easily volatile ruthenium (Ru) or expensive Rh. In addition, when supporting the latter rare earth element oxide and noble metal, the addition of the rare earth element makes it difficult for the transition metal ions at the B site that constitute the perovskite to be reduced in a reducing atmosphere, and there is a risk that it will not exhibit high activity. There is. As described above, conventional exhaust gas purifying catalysts have problems in terms of the required high activity and high temperature durability. In view of the above-mentioned problems, the present inventors focused on the combination of a complex oxide having a perovskite structure and a noble metal, and as a result of various studies, they arrived at the present invention. [Object of the Invention] An object of the present invention is to provide an exhaust gas purifying catalyst with excellent activity and heat resistance. [Structure of the Invention] The exhaust gas purifying catalyst of the present invention is a catalyst in which palladium is contained in a defective perovskite composite oxide having a specific surface area of 10 m ² /g or more, and 10 to 50% of the palladium is defective. It is characterized in that palladium is dissolved as a solid solution in the perovskite composite oxide, and the remaining palladium is supported in the defective perovskite composite oxide in the form of PdO or Pd. In the present invention, the defective perovskite composite oxide functions as a carrier and a catalyst component, and is represented by the general formula [A _1-x Ax′]BO ₂ . More specifically, La _1-x Sr _x MO ₃ (0.1<x<
0.3, M can be one or more of transition metals such as Co, Ni, Mn, Fe, and Cr). Perovskite composite oxide compositions that combine rare earth elements and transition metals have high activity, and when Sr is further added to this, oxygen vacancies are created, resulting in high activity. Note that among the above elements M, Co, Ni, and Mn are preferable because they are easily reduced. Furthermore, as the value of x approaches 0.2, the specific surface area tends to increase. Further, the specific surface area of the defective perovskite composite oxide needs to be 10 m ² /g or more. this is,
This is to improve the oxidation activity of CO and HC and to support palladium. Palladium contained in the defective perovskite composite oxide acts as a catalyst component, and its content is 0.06 to 0.185% by weight (hereinafter referred to as wt) based on the defective perovskite composite oxide.
%) is desirable. 0.06wt
If it is less than 0.185wt%, the NO _x purification ability is low, and if it exceeds 0.185wt%, the activity cannot be improved commensurate with the content. The most important thing in the present invention is the state of inclusion of palladium in the defective perovskite composite oxide. That is, 10 to 50% of palladium is substituted as a solid solution in the perovskite structure, and the remaining palladium is supported on the defective perovskite composite oxide in the form of PdO or Pd. This state of palladium content provides the excellent effects of the present invention in terms of activity and heat resistance. Note that the supported palladium is considered to be supported in a highly dispersed manner because the defective perovskite composite oxide has a large specific surface area. The action of palladium in this catalyst is not necessarily clear, but it is thought that the following phenomenon occurs. Defect perovskite composite oxide La _1-x Sr _x MO ₃
Taking the case where M is Co as an example, the defective perovskite composite oxide is La3+ 1-xSr2+ x
Co ³⁺ O _3- 〓 (〓 is the amount of oxygen vacancies), and by dissolving divalent Pd (Pd is stable in its divalent state) in this trivalent Co site, the following formula [a ], the amount of oxygen vacancies or Co ⁴⁺ produced increases. [La _1-x Sr _x ] [Co ³⁺ , Co ⁴⁺ , Pd ³⁺ ]
O _3- 〓′(〓′＞〓) [a] Therefore, active oxygen is easily generated,
Activity improves. On the other hand, in order to improve NO _x purification activity, it is important that Pd ²⁺ in solid solution coexist with Pd existing alone. The order of NO _x purification activity is (mixed state of fixed Pd and single Pd) > (state of single Pd) > (state of solid solution)
Pd state), and as in the present invention, solid solution Pd
When Pd is mixed with Pd alone, a significant improvement in activity can be obtained (see Examples). Note that although PdO is easily reduced to Pd in a reducing atmosphere, Pd ²⁺ dissolved in solid solution is difficult to reduce, and the activity can be improved by coexisting Pd and Pd ⁿ⁺ . In addition, at high temperatures around 900°C, sintering of Pd is suppressed due to the precipitation and re-solid solution phenomenon of Pd dissolved in solid solution in an atmosphere of repeated oxidation and reduction.
High temperature durability is improved. Furthermore, regarding the effects of the defective perovskite composite oxide and noble metals, only palladium improves _NOx purification activity due to a significant synergistic effect, while Rh and Pt have low activity. This is because Pd is divalent, Rh is trivalent, and Pt is quadrivalent and stable, and Rh has the same valence and ionic radius as M ions such as Co ³⁺ , so it is very easy to form a solid solution. Since Pt ⁴⁺ dissociates into Pt at around 600°C, it exists in a state with almost no solid solution. Therefore, neither Rh nor Pt exhibits the same effect as Pd described above. In the present invention, the amount of palladium substituted as a solid solution in the perovskite structure is set to 10 to 50% in order to maintain the mixed state amount of Pd and Pd ⁿ⁺ as Pd>Pd ⁿ⁺ under high-temperature usage conditions. be. It is considered that the catalytic activity is improved when the amount of supported palladium (Pd) is greater than the solid solution palladium (Pd ⁿ⁺ ). Examples of methods for producing the catalyst of the present invention include the following. The nitrates of each of the defective perovskite composite oxides were mixed in a predetermined stoichiometric ratio, and 0.1 mo.
Make an aqueous solution of
to form a coprecipitate. After this precipitate is then freeze-dried, 10-50% of the Pd content is
Impregnated with palladium nitrate equivalent to 110℃,
By drying in the atmosphere for 12 hours and then firing in the atmosphere at 800° C. for 5 to 10 hours, a single-phase defective perovskite composite oxide containing Pd as a solid solution is obtained. This powder is extremely fine with a particle size of less than 1μm, and has a particle size of 10m ² /
It has a specific surface area of g or more. Furthermore, the catalyst of the present invention is obtained by impregnating and supporting this powder with the remaining palladium nitrate aqueous solution and calcining it in the atmosphere at 600° C. for 3 hours. That is, when producing a defective perovskite composite oxide, by adding palladium to the raw material, it is possible to make palladium a solid solution in the composite oxide, and furthermore, palladium can be added to this by a normal supporting method. Palladium can be supported on the above composite oxide by supporting palladium. [Effect of the invention] The catalyst of the present invention reduces CO, HC and
It has excellent NO _x purification activity. In addition, _NO
There is no decrease in purification rate. Further, the catalyst of the present invention does not decrease in specific surface area or purification activity even when used for a long time at 900° C. or higher, and has excellent high-temperature durability. Furthermore, since the catalyst of the present invention suppresses the production of ammonia during use, there is no high-temperature alkali corrosion to the catalyst container. [Examples] Examples of the present invention will be described below. Example 1 First, 9.12 g of ammonium carbonate was dissolved in 100 c.c. of distilled water, and 38 c.c. of aqueous ammonia was further added to this aqueous solution to prepare a coprecipitation neutralizer. Next, 0.04 mol (17.32 g) of lanthanum nitrate, strontium nitrate
0.01mol (2.12g) and cobalt nitrate 0.05mol (20.2
g) was dissolved in distilled water of step 1, and the coprecipitation neutralizer was added dropwise to this aqueous solution to adjust the final pH to 9.0 to obtain a coprecipitate. This coprecipitate was aged for a day and night, then absorbed and filtered on paper and freeze-dried to obtain hydroxides of each component (lanthanum, strontium, cobalt). Add to this an aqueous palladium nitrate solution (0.054wt Pd) equivalent to 30% of the palladium content.
%) and dried in air at 110℃ for 12hr.
It was further fired in the air at 800°C for 5 hours. This results in
A single-phase composite oxide with a perovskite structure of La _0.8 Sr _0.2 (Co, Pd) O ₃ containing palladium in solid solution was obtained. Then, the palladium content is added to this composite oxide.
It was impregnated with an aqueous solution of palladium nitrate equivalent to 70% (0.126 wt% as Pd), dried in the air at 110°C for 12 hours, and then fired in the air at 600°C for 3 hours. As a result, 0.054wt% of palladium is dissolved in the perovskite structure, and 0.126wt% of palladium is dissolved in the perovskite structure.
A catalyst (sample No. 1) supported as PdO fine particles was obtained. Example 2 Pd content (total amount of solid-dissolved Pd and supported Pd) was set to 0, 0.0063, 0.0125, 0.0375,
Catalyst containing Pd (each sample
No. C1, 2 to 7) were prepared. Example 3 Sample No. 8 shown in Table 1 was prepared in the same manner as in Example 1 except that nickel nitrate, iron nitrate, or chromium nitrate was used instead of cobalt nitrate in Example 1.
~10 catalysts were prepared. Example 4 A mixture of ammonia water and ammonium carbonate with hydrogen peroxide added thereto was used as the coprecipitation neutralizing agent in Example 1, and manganese nitrate was used in place of cobalt nitrate, but the rest was the same as in Example 1. First
A catalyst of sample No. 11 shown in the table was prepared. Comparative Example 1 A coprecipitate was prepared in the same manner as in Example 1, and then dried in the atmosphere at 110°C for 12 hours without impregnating the coprecipitate with an aqueous palladium nitrate solution.
It was fired in the atmosphere at ℃ for 5 hours. As a result, a defective perovskite composite oxide of La _0.8 Sr _0.2 CoO ₃ in which Pd was not dissolved was formed. After that, this composite oxide was impregnated with an aqueous palladium nitrate solution having a Pd content of 0.18 wt%, dried in the air at 110°C for 12 hours, and then dried for 600 hrs.
℃, 3 hours in the atmosphere. As a result, a catalyst (sample No. C2) in which all palladium was supported in the form of PdO was prepared. Comparative Example 2 A coprecipitate was prepared in the same manner as in Example 1, and lyophilized to prepare a hydroxide. Thereafter, this hydroxide was impregnated with an aqueous palladium nitrate solution having a Pd content of 0.18 wt%, dried in the air at 110°C for 12 hours, and further calcined in the air at 800°C for 5 hours. As a result, all of the Pd was dissolved in the catalyst (sample No. C3).
was prepared. Comparative Example 3 Rhodium nitrate or chloroplatinic acid was used in place of palladium nitrate in Example 1, and the others were as follows:
Sample No. C4 shown in Table 1 in the same manner as Example 1,
A C5 catalyst was prepared. In addition, in sample Nos. C4 and C5, the content state of the contained metals (Rh, Pt) was unknown. Comparative Example 4 La _0.8 Sr _0.2 CoO ₃ was prepared in the same manner as in Comparative Example 1. Thereafter, this was impregnated with an aqueous solution of cerium nitrate so that the weight ratio of La _0.8 Sr _0.2 CoO ₃ and cerium oxide was 1:1, dried in the air at 110°C for 12 hours, and further dried in the air at 600°C for 3 hours. It was fired in As a result, a two-phase mixture of cerium oxide (CeO ₂ ) and a defective perovskite composite oxide of La _0.8 Sr _0.2 CoO ₃ was formed. After that, this composite oxide was impregnated with a palladium nitrate aqueous solution having a Pd content of 0.18 wt%, dried in the atmosphere at 110°C for 12 hours, and then dried for 600 hrs.
℃, 3 hours in the atmosphere. As a result, the above 2
A catalyst (sample No. C3) in which Pd was supported on the phase mixture was prepared. Examples 1 to 3 and Comparative Examples 1 to 3 above
Table 1 shows the catalysts prepared in . In addition, Pt-
Rh/Alumina catalyst (Pt-Rh content 0.2wt%)
is shown as sample No.C7, or Pd/alumina catalyst (Pd content 0.2wt%) is shown as sample No.C8. In addition, the state of the contained metals in both sample Nos. C7 and C8 was unknown.

[Table] (Test Example 1: Purification activity) Regarding the catalysts shown in Table 1 above, in the untreated state and in the atmosphere of automobile exhaust model gas (equivalence point composition)
Catalytic activity was measured as follows after the 900°C, 5-hour electric furnace durability test. To measure the catalytic activity, a gas having the composition shown in Table 2 is introduced into a catalyst bed filled with a catalyst at a space velocity (SV) of 30,000 hr ^-1 , and the temperature rise characteristics and air-fuel ratio (A/
F) By measuring the characteristics.

[Table] The temperature rise of the above temperature rise characteristic is determined by the temperature of the introduced gas at the equivalent point composition of A/F (A/F = 14.6, which is assumed to be the chemical equivalent ratio S = 1.0 in the following formula [b]). was heated at a constant rate, and the purification rates of NO, C ₃ H ₆ and CO were continuously determined. S = [NO] + 2 [O ₂ ] / [CO] + [H ₂ ] + 9・[C ₃ H ₆
] [b] (In the above formula, [ ] indicates the composition ratio in the gas flowing out from the catalyst layer.) In addition, the measurement of the A/F characteristics described above is performed by changing the catalyst layer temperature.
The purification rates of NO, CO, and C ₃ H ₆ were determined when the atmosphere was changed from rich to lean by keeping the temperature constant at 400°C and varying the amount of O ₂ in the gas composition shown in Table 2. The results of the above tests are summarized in the form shown in Table 3.
This is shown in Figures 1 to 12.

[Table] From Figure 1, regarding the NO purification ability, catalyst sample No. 1 of this example is compared to catalyst samples No. C2 and C3 of the comparative example,
It can be seen that it is superior to C6 and C7. From FIG. 2, it can be seen that even after the durability test, catalyst sample No. 1 of this example is superior to catalyst samples No. C6 and C7 of the comparative example in terms of NO purification ability. From FIG. 3 and FIG. 4, it can be seen that catalyst sample No. 1 of this example is superior to catalyst samples No. C6 and C7 of the comparative example in terms of CO and C ₃ H ₆ purifying ability. From Figures 5 and 6, even after the durability test
Catalyst sample of this example regarding CO and C ₃ H ₆ purification ability
It can be seen that No. 1 is superior to catalyst samples No. C6 and C7 of the comparative example. From FIG. 7, catalyst samples No. 2, 3, and
It can be seen that samples Nos. 4, 5, 6, and 7 are superior to the comparative catalyst sample No. C1 in terms of NO purification ability. Furthermore, the Pd content of samples No. 5, 6, and 7 is 0.06~
It can be seen that those in the range of 0.185wt% have better NO purification ability. From FIG. 8, catalyst samples No. 1, 8, and
Nos. 9, 10, and 11 have excellent NO purification ability, especially CO, as the B site metal of [A _1-x A _x ′]BO ₃ .
It can be seen that Ni and Mn have better NO purification ability. From Figure 9, catalyst sample No. 1 containing Pd is
It can be seen that the NO purification ability is superior to that of catalyst samples No. C4 and C5 containing Rh or Pt. From Figures 10 to 12, catalyst sample No. 1 of this example has the same purification ability as the Pt-Rh/alumina catalyst (No. C7), and the Pd/alumina catalyst (No. C8). It can be seen that the NO purification ability is higher in the rich region. (Test Example 2: Ammonia Production Amount) Three types of catalysts, Sample No. 1, No. C7, and No. C8 shown in Table 1, were A/F in the same condition as Test Example 1 without being treated. The characteristics were measured and the amount of by-product ammonia (NH ₃ ) produced was determined. The results are shown in FIG. Note that the amount of NH ₃ produced in the figure indicates the ratio (%) of the amount of NH ₃ produced to the amount of NO introduced. From FIG. 13, it can be seen that the catalyst sample No. 1 of this example produced less NH ₃ than the catalyst samples No. C7 and C8 of the comparative example. [Test Example 3: Specific surface area] Defected perovskite composite oxide La _1-x Sr _x CoO ₃
A catalyst supporting Pd was prepared in the same manner as in Example 1 by changing the Sr addition amount x to 0.02 and 0.5, and the specific surface area of the catalyst was measured by the BET method. The results are shown in FIG. From Figure 14, as the Sr addition amount x approaches 0.2,
It can be seen that the specific surface area of the catalyst increases.

[Brief explanation of the drawing]

Figures 1 to 9 are diagrams showing the purification rate curves of the temperature increase characteristic test of the catalyst in the example, and Figures 10 to 12 are diagrams showing the purification rate curve of the A/F characteristic test of the catalyst in the example. 13 is a diagram showing the ammonia production amount of the catalyst in the example, and FIG. 14 is a diagram showing the amount of La _1-x Sr _x CoO ₃ of the catalyst in the example.
FIG. 3 is a diagram showing changes in the specific surface area of the catalyst with respect to the amount of Sr added.

Claims (1)

[Claims] It is a catalyst in which palladium is contained in a defective perovskite composite oxide having a specific surface area of 1 10 m ² /g or more, and 10 to 50% of the palladium is dissolved in the defective perovskite composite oxide, and the remaining palladium An exhaust gas purifying catalyst characterized in that PdO or Pd is supported in a defective perovskite composite oxide.