JP2010119994A - Catalyst for cleaning exhaust - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the catalytic performance of a catalyst for cleaning exhaust including a Ce-Zr complex oxide supporting Pd and Rh employed therein as catalytic metal. <P>SOLUTION: The catalyst for cleaning exhaust includes a plurality of catalyst layers 2, 3 laid over a support 1. The plurality of the catalyst layers include a lower catalyst layer 2 comprising a first complex oxide containing Ce and Zr, and Pd supported by the first complex oxide, and an upper catalyst layer 3 comprising a second complex oxide containing Ce and Zr, and Rh supported by the second complex oxide. At least one species of alkali earth metals selected from Ca, Sr, and Mg is dissolved as a solid solution at least in one of the first complex oxide and the second complex oxide. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an exhaust gas purification catalyst.

  A Ce-containing oxide is often added to the exhaust gas purification catalyst. For example, for a three-way catalyst that purifies HC (hydrocarbon), CO (carbon monoxide), and NOx (nitrogen oxide), the Ce-containing oxide occludes oxygen in an oxygen-excess atmosphere than stoichiometry, resulting in a lack of oxygen. It is known that the A / F window (air-fuel ratio region) in which the catalyst works effectively is expanded by releasing oxygen in the atmosphere. In an exhaust gas purifying catalyst for a lean combustion engine such as a diesel engine, it is known that a Ce-containing oxide adsorbs a relatively large amount of NOx contained in the exhaust gas. In the exhaust gas purification catalyst for particulate combustion supported on a diesel particulate filter, the Ce-containing oxide takes in oxygen in the exhaust gas by changing the valence of Ce ions and releases the internal oxygen as active oxygen It is known to cause an oxygen exchange reaction that promotes the burning of particulates by the active oxygen.

  By the way, ceria which is a typical Ce-containing oxide has low heat resistance. Therefore, in order to improve the heat resistance, a CeZr-based composite oxide in which Zr is dissolved in ceria has been developed. More recently, in order to improve the catalyst performance, a catalytic metal is dissolved in the CeZr-based composite oxide. It is also done.

For example, Patent Document 1 describes that a composite oxide containing a rare earth metal such as Ce, an alkaline earth metal such as Ba, Zr, and a noble metal is used as an exhaust gas purification catalyst. . For example, a composite oxide represented by (Ce, Zr) O ₂ , a composite oxide represented by BaCeO ₃ , a composite oxide represented by BaZrO ₃ , and a Ba (Zr, Ce) O ₃ A powder containing Pt in a solid solution is disclosed.

  However, the composite oxide containing Ba is obtained by adding the CeZr composite oxide to a barium acetate solution and drying and firing the resulting slurry. Moreover, the Example about complex oxide containing alkaline-earth metal other than Ba is not disclosed. An example in which Pd or Rh is supported on the powder is not disclosed.

Patent Document 2 discloses an exhaust gas purifying catalyst in which Pt is supported on a composite oxide of Zr and at least one metal element selected from rare earth metals and alkaline earth metals and containing Ce. The use is described. For example, an exhaust gas purification catalyst of Pt / CeO ₂ —ZrO ₂ —Y ₂ O ₃ —BaO is disclosed. As the alkaline earth metal, Mg, Ca, Ba are preferred from the viewpoint of strong interaction with noble metals and their oxides and high affinity, and such alkaline earth metals with low electronegativity are preferred. Since the element has a strong interaction with the noble metal, it binds to the noble metal through oxygen in an oxidizing atmosphere, suppresses the transpiration and sintering of the noble metal, and can sufficiently suppress the deterioration of the noble metal that is the active site. Has been.

However, the complex oxide containing Ba is obtained by impregnating a CeO ₂ —ZrO ₂ —Y ₂ O ₃ complex oxide with a barium acetate solution and baking it. Moreover, the Example about complex oxide containing alkaline-earth metal other than Ba is not disclosed. An example in which Pd or Rh is supported on a composite oxide is not disclosed.
JP 2006-346587 A JP 2007-289921 A

  As described above, complex oxides containing Ba are known, and it is also known in Patent Document 2 that noble metals are supported on such complex oxides. However, when the CeZr-based composite oxide is impregnated with a Ba solution and fired, Ba having a large ionic radius is supported on the surface of the CeZr-based composite oxide particles as Ba oxide, or Ba and Ce or Zr. It is not considered to be dispersed as a solid oxide in the particles. Further, even if it is a noble metal, the catalytic function differs depending on the type, and Pd changes between the oxidation state (PdO) and the reduction state (metal Pd), thereby contributing mainly to the oxidation purification of HC and CO, and Rh Contributes mainly to the reduction and purification of NOx when in the reduced state (metal Rh).

  Therefore, the catalyst configuration in which Pt is bonded via oxygen to Ba having a low electronegativity as described in Patent Document 2, that is, the Pt oxide is formed on the surface of the composite oxide particle is appropriate for Pt. Even so, Pd and Rh cannot always be preferred.

  Accordingly, the present invention relates to an exhaust gas purification catalyst that employs Pd and Rh as catalyst metals and supports them on a CeZr-based composite oxide, and an object thereof is to improve the catalyst performance.

  In the present invention, in order to solve the above problems, an alkaline earth metal is dissolved in the CeZr-based composite oxide.

That is, the present invention is an exhaust gas purifying catalyst comprising a plurality of catalyst layers stacked on a carrier,
As the plurality of catalyst layers, a first composite oxide containing Ce and Zr, a lower catalyst layer containing Pd supported on the first composite oxide, and a second composite oxide containing Ce and Zr And an upper catalyst layer containing Rh supported on the second composite oxide,
At least one alkaline earth metal selected from Ca, Sr and Mg is dissolved in at least one of the first composite oxide and the second composite oxide.

  Therefore, according to the present invention, at least one of the first composite oxide and the second composite oxide acts so that the alkaline earth metal promotes oxygen in / out, and the alkaline earth metal When the crystal distortion increases due to the solid solution, oxygen storage / release performance and oxygen exchange reactivity increase.

Thus, an important feature of the present invention is that the NOx adsorption property of the composite oxide is improved by the solid solution of the alkaline earth metal, and the amount of NOx desorbed in a low temperature region is increased. . That is, NOx is adsorbed as NO ₂ or NO ₃ ^{− on} the composite oxide, but is desorbed as NO, so that oxygen remains adsorbed on the particle surface of the composite oxide after NO is desorbed. Thus, the oxidation ability or activity of the surface of the composite oxide particles is increased even in a low temperature range.

  As a result, in the case where the alkaline earth metal is dissolved in the first composite oxide, Pd is oxidized by the highly active oxygen released from the first composite oxide even in an atmosphere where oxygen is insufficient compared to stoichiometry. In addition, even in an oxygen-excess atmosphere as compared with stoichiometry, electron donation from an alkaline earth metal having a low electronegativity to Pd causes the Pd to increase its electron density and share with oxygen (O) atoms. The bondability becomes stronger and it becomes easier to keep the oxidized state. For this reason, Pd in the oxidized state tries to return to the oxidized state by reducing NOx even though it is once reduced by oxidizing HC and CO. That is, Pd is likely to undergo a state change between PdO (oxidized state) and metal Pd (reduced state), and a high activity state is maintained. As a result, even in a relatively low temperature range, oxidation of HC and CO and reduction of NOx proceed efficiently and early activation of the catalyst can be achieved.

  On the other hand, in the case where the alkaline earth metal is dissolved in the second composite oxide, the metal Rh in a reduced state in an atmosphere in which oxygen is insufficient as compared with stoichiometry has an electronegativity even in an oxygen-excess atmosphere. It is easy to maintain a reduced state with high activity due to the action of an alkaline earth metal having a small NO, and even in a relatively low temperature range, NOx by Rh is present on the surface of the composite oxide particle that has become highly active due to NOx desorption. The reduction of the catalyst proceeds efficiently, and the catalyst can be activated early.

  The alkaline earth metal is preferably at least one selected from Ca, Sr and Mg having a smaller ionic radius than Ba. In the case of Ba, since the ionic radius is larger than that of Ca, Sr and Mg, it is difficult to dissolve in the first and second composite oxides, and the effect of increasing the basicity of the composite oxide is low. This is because it is difficult to make an oxidized state or a reduced state of Rh.

Both the first composite oxide and the second composite oxide contain Zr as a main component, and the mass ratio of ZrO ₂ to each CeO ₂ is larger in the second composite oxide than in the first composite oxide. Is preferred. That is, the composite oxide containing Ce and Zr has higher heat resistance as the ZrO ₂ content increases. Thus, the upper catalyst layer is more easily heated by the exhaust gas than the lower catalyst layer, and the second composite oxide of the upper catalyst layer has high heat resistance, so that it is difficult to cause thermal degradation. Is protected from the heat of the exhaust gas by the upper catalyst layer, so that thermal deterioration hardly occurs, and the durability of the catalyst as a whole becomes high.

  As described above, according to the present invention, Pd is supported on the first composite oxide containing Ce and Zr in the lower catalyst layer, and Rh is added to the second composite oxide containing Ce and Zr in the upper catalyst layer. And a catalyst structure in which at least one alkaline earth metal selected from Ca, Sr and Mg is dissolved in at least one of the first composite oxide and the second composite oxide is employed. From at least one of the first composite oxide and the second composite oxide, the oxygen storage / release performance and the oxygen exchange reactivity are increased, and the NOx desorption amount is increased in the low temperature range, thereby increasing the surface of the composite oxide particles. As a result, the exhaust gas purification performance increases.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. It should be noted that the following description of the preferred embodiment is merely illustrative in nature, and is not intended to limit the present invention, its application, or its use.

  In FIG. 1, reference numeral 1 denotes a cell wall of a honeycomb carrier constituting a three-way catalyst for purifying automobile exhaust gas, and a lower catalyst layer 2 and an upper catalyst layer 3 are laminated on the surface thereof. The lower catalyst layer 2 contains first composite oxide particles containing Ce and Zr and Pd supported on the first composite oxide particles, and the upper catalyst layer 3 contains a first composite oxide particle containing Ce and Zr. Two composite oxide particles and Rh supported on the second composite oxide particles are contained. An alkaline earth metal is dissolved in at least one of the first composite oxide and the second composite oxide.

<Oxygen storage and release performance>
Pd / CZN catalyst powder in which Pd is supported on a composite oxide CZN containing Ce, Zr, and Nd, and Pd is supported on a composite oxide CZN-Ca in which Ca as an alkaline earth metal is dissolved in the CZN. A Pd / CZN—Ca catalyst powder was prepared, and a test catalyst was prepared in which each was supported on a cordierite honeycomb carrier (capacity: 25 mL). The supported amount of CZN and CZN-Ca is 73 g / L, and the supported amount of Pd in the catalyst powder is 2.7% by mass with respect to the composite oxide. Each composite oxide of CZN and CZN-Ca was prepared by a coprecipitation method. Hereinafter, the preparation method of CZN-Ca is demonstrated.

That is, a predetermined amount of each of zirconium oxynitrate, cerium nitrate, neodymium (III) nitrate, and calcium nitrate was mixed with water to obtain a raw material solution (acidic). 7% ammonia water as a basic solution is added to this raw material solution (other basic solutions such as caustic soda aqueous solution can also be used), and the cloudy solution is left for a day and night, and the resulting cake is centrifuged. And washed thoroughly with water. The cake washed with water was dried at a temperature of about 150 ° C. and then calcined under the condition that it was kept at a temperature of 400 ° C. for 5 hours and then kept at a temperature of 500 ° C. for 2 hours. CZN—Ca obtained as described above has a structure in which Ca is arranged in the crystal lattice, interatomic or oxygen deficient portion of the composite oxide. The composition ratio of CZN—Ca excluding Ca is CeO ₂ : ZrO ₂ : Nd ₂ O ₃ = 23: 67: 10 (mass ratio). The solid solution amount of Ca was 10% by mass of CZN—Ca in terms of CaO.

The above CZN was prepared by the same coprecipitation method without adding calcium nitrate in the preparation method of CZN-Ca, and the composition ratio was CeO ₂ : ZrO ₂ : Nd ₂ O ₃ = 23: 67: 10. (Mass ratio).

After performing aging (O ₂ ; 2% by mass, H ₂ O; 10% by mass, remaining: N ₂ atmosphere at a temperature of 800 ° C. for 24 hours) on each of the above test catalysts, the oxygen storage / release performance was measured. Examined. That is, a test catalyst is attached to a fixed bed gas flow device, an O ₂ sensor is arranged before and after the catalyst, and a simulated exhaust gas of A / F = 14.4 (Stoichi) is first flowed, and the simulated exhaust gas. The response of the change in the A / F value after the catalyst (downstream side) to the change in the A / F value before the catalyst (upstream side) when the A is changed to A / F = 13.2 (rich) It was. The simulated exhaust gas temperature was 450 ° C., and the space velocity was 60000 h ⁻¹ . The results are shown in FIG.

  According to the figure, in all of the test catalysts, the A / F value after the catalyst changes with a delay in response to the decrease in the A / F value before the catalyst. Is different. That is, the P / CZN-Ca containing Ca has a greater response delay than the Pd / CZN not containing Ca, and the A / F value after the catalyst changes downward. A large response delay means that oxygen is rapidly released from the CeZr-based composite oxide particles when the oxygen concentration in the atmosphere is lowered, that is, the oxygen release rate is fast.

  The difference in the A / F value before and after the catalyst after the A / F is switched from 14.4 to 13.2 is an indicator of the oxygen release rate. Therefore, when the difference between the A / F values before and after the catalyst for 1 second from the A / F switching is integrated, Pd / CZN-Ca = 4.88 and Pd / CZN = 0.58. The value of Pd / CZN-Ca is about 1.07 times that of Pd / CZN. That is, it can be said that the oxygen release rate of Pd / CZN—Ca is about 1.07 times that of Pd / CZN. This difference is an effect due to the solid solution of Ca in the CeZr-based composite oxide.

  Further, according to FIG. 2, even when the difference between the A / F values from the switching of the A / F until the post-catalyst A / F value becomes substantially equal to the pre-catalyst A / F value is integrated, Since it is clear that the integrated value of Pd / CZN—Ca is larger than that of Pd / CZN, the catalyst powder of Pd / CZN—Ca is obtained by solid solution of Ca into CeZr-based composite oxide particles. It is recognized that the oxygen storage amount is also increasing.

<NOx adsorption / desorption performance>
A Pd / CZN—Ca catalyst powder having a CaO content ratio of 3 mass% and a Pd / CZN catalyst powder were prepared. The composition ratio of those CZNs is CeO ₂ : ZrO ₂ : Nd ₂ O ₃ = 23: 67: 10 (mass ratio). Furthermore, a Rh / CZN catalyst powder in which Rh is supported on a composite oxide CZN having a composition ratio of CeO ₂ : ZrO ₂ : Nd ₂ O ₃ = 10: 80: 10 (mass ratio), and CZN having the same composition ratio An Rh / CZN-Ca catalyst powder in which Rh is supported on a composite oxide CZN-Ca in which Ca is solid-solved was prepared. In both cases, the amount of Rh supported is 1.2% by mass with respect to each of CZN and CZN-Ca. Moreover, the solid solution amount of Ca in the Rh / CZN-Ca catalyst powder was 3% by mass of the entire CZN-Ca in terms of CaO.

Thus, the NOx adsorption / desorption performance of the four types of catalyst powders was examined. First, while supplying reducing gas (H ₂ ; 0.45%, residual He, flow rate: 100 mL / min) to 50 mg of catalyst powder, the gas temperature was increased from room temperature (25 ° C.) at a rate of 30 ° C./min. And kept at a temperature of 600 ° C. for 10 minutes, and then the gas temperature was returned to room temperature. Next, NO-containing gas (NO; 4000 ppm, O ₂ ; 3.0%, remaining He, flow rate; 100 mL / min) is supplied to the catalyst powder for 15 minutes at room temperature, and then He gas (flow rate; 100 mL / min) is supplied. The NOx was desorbed from the catalyst powder by increasing the gas temperature to 600 ° C at a rate of 20 ° C / min. The results are shown in FIGS.

  According to FIG. 3, in the case of Pd / CZN—Ca, the NO desorption amount at 210 ° C. or less is much higher than that of Pd / CZN, and when it exceeds 210 ° C., the NO desorption amount is reversed. The amount of NO desorbed from Pd / CZN-Ca is small. According to FIG. 4, in the case of Rh / CZN-Ca and Rh / CZN, the NO desorption amount in the vicinity of 290 ° C. is substantially the same, but the temperature range lower than 290 ° C. and the temperature range higher than 290 ° C. (450 Rh / CZN-Ca has a much larger NO desorption amount than Rh / CZN.

  In the case of Pd / CZN-Ca, the total NO adsorption amount is 364 a.u., and the NO desorption amount up to 230 ° C. is 19.6% of the total NO adsorption amount, that is, 71.2 a.u. It was. On the other hand, in the case of Pd / CZN, the total NO adsorption amount is 390 a.u., and the NO desorption amount up to 230 ° C. is 3.2% of the total NO adsorption amount, that is, 12.6 a. u.

  On the other hand, in the case of Rh / CZN-Ca, the total NO adsorption amount is 972 a.u., and the NO desorption amount up to a temperature of 230 ° C. is 45.3% of the total NO adsorption amount, that is, 440 a.u. It was. On the other hand, in the case of Rh / CZN, the total NO adsorption amount is 608 a.u., and the NO desorption amount up to a temperature of 230 ° C. is 34.5% of the total NO adsorption amount, that is, 210 a.u. Met.

  From the above, it can be seen that when Ca is dissolved in CeZr-based composite oxide particles, the amount of NO desorption at least in the low temperature region increases. Further, when Rh is supported on CeZr-based composite oxide particles in which Ca is dissolved, the amount of NO desorption in a high temperature region is also increased.

Here, NOx in the exhaust gas is adsorbed on the CeZr-based composite oxide particles as NO ₂ or NO ₃ ⁻ , but becomes NO when desorbed, so that oxygen is present on the surface of the composite oxide particles. The remaining adsorbed state is obtained, and the oxidation ability or activity of the particle surface is increased. Therefore, both Pd / CZN-Ca and Rh / CZN-Ca have higher oxidation ability or activity on the surface of the composite oxide particles in a low temperature range, promote oxidative purification of HC and CO, and promote early activity of the catalyst. Can be achieved. In addition, in the case of Rh / CZN-Ca, since active desorption of NO is observed even in the vicinity of 300 ° C. to 400 ° C., the oxidation ability or activity on the surface of the composite oxide particles is high even after light-off of the catalyst. It is estimated that CO and NOx are efficiently purified.

<Exhaust gas purification performance>
Catalysts of Examples 1 to 9 and Comparative Examples 1 to 3 in which various catalyst powders were applied to the lower catalyst layer 2 and the upper catalyst layer 3 shown in FIG. 1 were prepared. In addition, as a carrier for these catalysts, a honeycomb carrier made of cordierite having a cell wall thickness of 3.5 mil (8.89 × 10 ⁻² mm) and 600 cells per square inch (645.16 mm ² ) (capacity) 1L) was used.

Example 1
In the lower catalyst layer 2, Pd / 23CZN-3% Ca as a Pd-supported OSC material and Pd-supported alumina were mixed and arranged. Pd / 23CZN-3% Ca has a composition ratio of CeO ₂ : ZrO ₂ : Nd ₂ O ₃ = 23: 67: 10 (mass ratio), and Ca is a solid solution of 3 mass% in terms of CaO. This is a catalyst powder in which 2.7% by mass of Pd is supported on an oxide (23CZN-3% Ca). Pd-supported alumina is a catalyst powder in which 8.6% by mass of Pd is supported on activated alumina. The supported amount of each catalyst powder per liter of the carrier is 75 g / L for Pd / 23CZN-3% Ca and 65 g / L for Pd-supported alumina.

In the upper catalyst layer 3, Rh / 10CZN as the Rh-supported OSC material and Rh-supported ZrO ₂ coated alumina were mixed and arranged. Rh / 10CZN is a catalyst powder obtained by supporting 1.2% by mass of Rh on a composite oxide (10CZN) having a composition ratio of CeO ₂ : ZrO ₂ : Nd ₂ O ₃ = 10: 80: 10 (mass ratio). It is. Rh-supported ZrO ₂ -coated alumina is obtained by supporting 1.2% by mass of Rh on a support material obtained by coating the surface of activated alumina particles containing 4% by mass of La ₂ O ₃ with 10% by mass of zirconium oxide. Catalyst powder. The supported amount of each catalyst powder per liter of support is 35 g / L for Rh / 10CZN and 25 g / L for Rh-supported ZrO ₂ -coated alumina.

-Example 2-
The same structure as in Example 1 was used except that the Pd-supported OSC material of the lower catalyst layer 2 was changed to Pd / 23CZN-3% Sr. Pd / 23CZN-3% Sr has a composition ratio of CeO ₂ : ZrO ₂ : Nd ₂ O ₃ = 23: 67: 10 (mass ratio), and Sr is a solid solution of 3 mass% in terms of SrO. This is a catalyst powder in which 2.7% by mass of Pd is supported on an oxide (23CZN-3% Sr).

-Example 3-
The same configuration as in Example 1 except that the Pd-supported OSC material of the lower catalyst layer 2 was Pd / 23CZN and the Rh-supported OSC material of the upper catalyst layer 3 was Rh / 10CZN-3% Ca. Pd / 23CZN is a catalyst powder in which 2.7% by mass of Pd is supported on a complex oxide (23CZN) of CeO ₂ : ZrO ₂ : Nd ₂ O ₃ = 23: 67: 10 (mass ratio). Rh / 10CZN-3% Ca has a composition ratio of CeO ₂ : ZrO ₂ : Nd ₂ O ₃ = 10: 80: 10 (mass ratio), and Ca is a solid solution of 3 mass% in terms of CaO. This is a catalyst powder in which 1.2% by mass of Rh is supported on an oxide (10CZN-3% Ca).

Example 4
The same configuration as in Example 3 except that the Rh-supported OSC material of the upper catalyst layer 3 was Rh / 10CZN-6% Ca. Rh / 10CZN-6% Ca is a catalyst powder having a Ca solid solution amount of 6% by mass in terms of CaO in the above Rh / 10CZN-3% Ca.

-Example 5
The same configuration as in Example 3 was used except that the Rh-supported OSC material of the upper catalyst layer 3 was Rh / 10CZN-9% Ca. Rh / 10CZN-9% Ca is a catalyst powder in which the amount of solid solution of Ca in the above Rh / 10CZN-3% Ca is 9 mass% in terms of CaO.

-Example 6
The same configuration as in Example 3 was used except that the Rh-supported OSC material of the upper catalyst layer 3 was Rh / 10CZN-3% Mg. Rh / 10CZN-3% Mg is a catalyst powder in which, in the above Rh / 10CZN-3% Ca, Mg is converted to MgO instead of Ca and dissolved in 3% by mass.

-Example 7-
The configuration was the same as that of Example 3 except that the Rh-supported OSC material of the upper catalyst layer 3 was Rh / 10CZN-3% Sr. Rh / 10CZN-3% Sr is a catalyst powder in which 3% by mass of Sr is converted into SrO instead of Ca in the above Rh / 10CZN-3% Ca.

-Example 8-
The same configuration as in Example 1 was adopted except that the Rh / 10CZN-3% Ca was used as the Rh-supported OSC material of the upper catalyst layer 3.

-Example 9-
The same configuration as in Example 1 except that Pd / 10CZN-3% Ca is adopted as the Pd-supported OSC material of the lower catalyst layer 2, and Rh / 23CZN-3% Ca is adopted as the Rh-supported OSC material of the upper catalyst layer 3. did. Pd / 10CZN-3% Ca is a catalyst powder in which 2.7% by mass of Pd is supported on a 10CZN-3% Ca composite oxide. Rh / 23CZN-3% Ca is a catalyst powder in which 1.2% by mass of Rh is supported on a 23CZN-3% Ca composite oxide.

-Comparative Example 1-
The same configuration as in Example 1 was adopted except that the Pd / 23CZN was adopted as the Pd-supported OSC material of the lower catalyst layer 2.

-Comparative Example 2-
The same configuration as in Example 1 was adopted except that Pd / Ca impregnated 23CZN was adopted as the Pd-supported OSC material of the lower catalyst layer 2. Pd / Ca-impregnated 23CZN is a composite oxide (Ca-impregnated 23CZN) obtained by impregnating 23CZN composite oxide with 3% by mass of calcium nitrate solution converted to CaO and drying and firing. Catalyst powder.

-Comparative Example 3-
The same configuration as in Example 3 was adopted except that Rh / Ca-impregnated 10CZN was adopted as the Rh-supported OSC material of the upper catalyst layer 3. Rh / Ca-impregnated 10CZN is a composite oxide (Ca-impregnated 10CZN) obtained by impregnating 10CZN composite oxide with 3% by mass of calcium nitrate solution converted to CaO and drying and firing. Catalyst powder.

-Comparative Example 4-
The lower catalyst layer 2 has the same configuration as that of Example 1, except that the Pt-supported OSC material is used instead of the Pd-supported OSC material. The Pt-supported OSC material is a catalyst powder in which 2.7% by mass of Pt is supported on 23CZN-3% Ca.

-Comparative Example 5-
The same configuration as in Example 1 was adopted except that Pd / 23CZN-3% Ba was adopted as the Pd-supported OSC material of the lower catalyst layer 2. Pd / 23CZN-3% Ba has a composition ratio of CeO ₂ : ZrO ₂ : Nd ₂ O ₃ = 23: 67: 10 (mass ratio), and Ba is a solid solution of 3 mass% in terms of BaO. This is a catalyst powder in which 2.7% by mass of Pd is supported on an oxide (23CZN-3% Ba).

[Evaluation of exhaust gas purification performance]
Each catalyst of Examples 1-9 and Comparative Examples 1-3 was subjected to bench aging treatment. This is because each catalyst is attached to the engine exhaust system, (1) A / F = 14 exhaust gas flows for 15 seconds → (2) A / F = 17 exhaust gas flows for 5 seconds → (3) A / F = 14.7 The exhaust gas flowing for 40 seconds is repeated for a total of 50 hours, and the engine is operated so that the catalyst inlet gas temperature is 800 ° C.

Thereafter, a core sample with a carrier volume of 25 mL was cut out from each catalyst, and this was attached to a model gas flow reactor, and a light-off temperature T50 (° C.) relating to purification of HC, CO, and NOx was measured. T50 (° C.) is the gas temperature at the catalyst inlet when the model gas temperature flowing into the catalyst is gradually increased from room temperature and the purification rate reaches 50%. The model gas was A / F = 14.7 ± 0.9. That is, the A / F is forced at an amplitude of ± 0.9 by adding a predetermined amount of fluctuation gas in a pulse form at 1 Hz while constantly flowing the main stream gas of A / F = 14.7. Vibrated. The space velocity SV is 60000 h ⁻¹ , and the heating rate is 30 ° C./min. The gas composition when A / F = 14.7, A / F = 13.8 and A / F = 15.6 is shown in Table 1, and the measurement result of the light-off temperature T50 is shown in Table 2. In Table 2, only the type of OSC material is described in each of the “Pd-supported OSC material” column of the lower catalyst layer and the “Rh-supported OSC material” column of the upper catalyst layer.

  When examining the CeZr-based composite oxide supporting Pd of the lower catalyst layer, in Examples 1 and 2, the light-off temperature T50 is lower by several tens of degrees C. than in Comparative Example 1, and the light of Example 1 The decrease in the off temperature T50 is particularly large. From this, it can be seen that when Ca or Sr is dissolved in the CeZr-based composite oxide, the catalyst can be activated early, and the effect of the solid solution is particularly large for Ca. In Comparative Example 2, CeZr-based composite oxide is impregnated and supported with Ca, but the light-off temperature is higher than that of Comparative Example 1. From this and the results of Example 1, it was found that the alkaline earth metal was not effective for the early activation of the catalyst only by being supported on the surface of the CeZr-based composite oxide particles, and the alkaline earth metal could be used to obtain the effect. It turns out that the metal needs to be dissolved. Further, Comparative Example 5 is obtained by dissolving Ba in a CeZr-based composite oxide, but the light-off temperature T50 is not significantly different from that of Comparative Example 1. This is presumably because Ba has a low effect of increasing its basicity by solid solution in the CeZr-based composite oxide.

  Further, when examining the noble metal supported on the CeZr-based composite oxide of the lower catalyst layer, Comparative Example 4 is one in which Pt is supported on a CeZr-based composite oxide in which Ca is dissolved, and the light-off temperature thereof. T50 is higher than any of Examples 1 to 9 and Comparative Examples 1 to 3 and 5, which employ Pd as a noble metal. This is presumably because in the case of Pt, it was stabilized in an oxidized state combined with oxygen (O) by the electron donating effect of the alkaline earth metal, and it was difficult to express the reduced state which is an active state. That is, with Pt, the effect of improving the catalytic activity due to the solid solution of the alkaline earth metal in the CeZr-based composite oxide cannot be obtained.

  Next, when examining the CeZr-based composite oxide supporting Rh of the upper catalyst layer, the light-off temperature T50 in Examples 3 to 7 is lower by several tens of degrees C than in Comparative Example 1. Therefore, it can be seen that when Ca, Sr or Mg is dissolved in the CeZr-based composite oxide, the catalyst can be activated early. In Comparative Example 3, CeZr-based composite oxide was impregnated and supported with Ca, but the light-off temperature was higher than that of Comparative Example 1. From this and the result of Example 3, it was found that the alkaline earth metal was not effective for the early activation of the catalyst only by being supported on the surface of the CeZr-based composite oxide particles. It turns out that the metal needs to be dissolved.

  Moreover, from the comparison of Examples 3, 6, and 7, the decrease in the light-off temperature T50 of Example 3 in which Ca was dissolved was the largest, followed by Example 7 in which Sr was dissolved. When compared with the CaO solid solution amount of Examples 3 to 5, the light-off temperature of Example 4 having a CaO solid solution amount of 6 mass% is the lowest, but there is no significant difference between Examples 3 to 5. From this, it can be seen that the early activation of the catalyst can be reliably achieved when the solid solution amount of the alkaline earth metal is converted to oxide to 3 mass% or more and 9 mass% or less.

In both Examples 8 and 9, Ca was dissolved in the CeZr-based composite oxide of each of the lower catalyst layer and the upper catalyst layer. In Example 8, ZrO _{2 with} respect to CeO ₂ of the composite oxide of the lower catalyst layer. The mass ratio (80/10) of the composite oxide in the upper catalyst layer is larger than the mass ratio (67/23) of the upper catalyst layer. In Example 9, on the contrary, the same mass ratio of the lower catalyst layer is higher in the upper catalyst layer. It is larger than the same mass ratio of the layers. Thus, the light-off temperature T50 is lower by several tens of degrees Celsius in Example 8 than in Example 9.

This is because the upper catalyst layer is more easily heated by exhaust gas than the lower catalyst layer. In the upper catalyst layer of Example 8, the CeZr-based composite oxide has a large ZrO ₂ mass ratio, and therefore its heat resistance is high. Since it is high, the thermal deterioration due to the above-mentioned bench aging becomes mild, and it is recognized that the lower catalyst layer is protected from the heat of the exhaust gas by the upper catalyst layer, and the thermal deterioration becomes mild.

1 is a cross-sectional view of an exhaust gas purifying catalyst according to the present invention. It is a graph which shows the change of A / F after a catalyst when A / F before a catalyst is changed about each of Pd / CZN-Ca catalyst powder and Pd / CZN catalyst powder. It is a graph which shows NO adsorption / desorption characteristic of each of Pd / CZN-Ca catalyst powder and Pd / CZN catalyst powder. It is a graph which shows the NO adsorption / desorption characteristic of each Rh / CZN-Ca catalyst powder and Rh / CZN catalyst powder.

Explanation of symbols

1 Support 2 Lower catalyst layer 3 Upper catalyst layer

Claims (2)

An exhaust gas purifying catalyst comprising a plurality of catalyst layers stacked on a carrier,
As the plurality of catalyst layers, a first composite oxide containing Ce and Zr, a lower catalyst layer containing Pd supported on the first composite oxide, and a second composite oxide containing Ce and Zr And an upper catalyst layer containing Rh supported on the second composite oxide,
At least one alkaline earth metal selected from Ca, Sr, and Mg is dissolved in at least one of the first composite oxide and the second composite oxide. catalyst. In claim 1,
Both the first composite oxide and the second composite oxide contain Zr as a main component, and the mass ratio of ZrO ₂ to each CeO ₂ is larger in the second composite oxide than in the first composite oxide. An exhaust gas purifying catalyst characterized by.

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