JP4397155B2 - Exhaust gas purification catalyst and method for producing the catalyst - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust gas purification catalyst and a method for producing the same.
[0002]
[Prior art]
A three-way catalyst is known as a catalyst for purifying exhaust gas of a spark ignition engine. In general, Pt and Rh as active metals, alumina as a support material that stabilizes the active metal and expands a contact area with gas to improve purification performance, and oxygen storage material (OSC) It is often composed of ceria (co-catalyst).
[0003]
On the other hand, in Patent Document 1, a component in which Rh is supported on an Nd—Zr composite oxide and a component in which Pt is supported on alumina or ceria are mixed and coated on a carrier, and then the coating layer is coated. Describes that 20 g of BaO is supported per liter of the carrier by impregnating with Ba solution. Moreover, it describes that the heat resistance of Rh improves by that.
[0004]
[Patent Document 1]
JP 09-141098 (paragraphs 0020, 0054, 0060)
[0005]
[Problems to be solved by the invention]
However, ceria generally has a problem of low heat resistance. For this reason, when the catalyst is exposed to a high temperature due to high-load operation of the engine, the oxygen storage capacity decreases, resulting in a significant decrease in catalyst performance, particularly low-temperature activity related to HC (hydrocarbon) purification. In addition, Rh is active in purifying HC, CO, and NOx (nitrogen oxide) 3 components in a small amount, and is particularly excellent in NOx purification performance. When this Rh is supported on ceria, its oxidative deterioration is prevented. That is, it is advantageous for maintaining its activity. However, as mentioned above, since the heat resistance of ceria is low, after the catalyst is exposed to a high temperature, Rh becomes unstable, causing sintering and the like and not effectively used for purification of exhaust gas. There's a problem.
[0006]
Further, when P (phosphorus) is contained in the engine oil, this P generates a glassy compound and covers the catalyst surface, impedes gas diffusion into the catalyst, and lowers its purification performance. There is a problem. Further, this P has a problem that it forms a compound with alumina or ceria and degrades them, and further combines with Pt as an active component to degrade it.
[0007]
The present invention intends to solve the above-mentioned problem of heat resistance of ceria, the problem of sintering of Rh, and the problem of P poisoning.
[0008]
[Means for Solving the Problems]
The present invention utilizes a Ce / Zr / Nd composite oxide and Ba to solve such problems.
[0009]
First, the invention according to claim 1 is an exhaust gas purifying catalyst containing Rh and Ba.
A plurality of catalyst layers are formed in layers on the support,
The catalyst layer located inside of the plurality of catalyst layers contains a component in which Pd is supported on alumina, an oxygen storage material, and Ba,
The catalyst layer located outside the inner catalyst layer contains a component in which the Rh and Ba are supported on a Ce / Zr / Nd composite oxide ,
The amount of Ba supported per liter of the carrier is 5 to 30 g .
[0010]
That is, Ce · Zr · Nd composite oxide (double oxide or mixed oxide) works as an oxygen storage material like ceria, but has higher heat resistance than ceria, and after the catalyst is exposed to high temperature. However, the crystal does not become so coarse. For this reason, a high oxygen storage capacity is maintained. That is, the air-fuel ratio window that can purify HC, CO, and NOx in the three-way catalyst with high efficiency is not narrowed, and high purification performance is maintained. At the same time, the activity of Rh supported on the Ce · Zr · Nd composite oxide is also maintained, which is advantageous for the purification of HC, CO and NOx.
[0011]
Further, in the catalyst according to the present invention, Ba is supported together with Rh on the Ce · Zr · Nd composite oxide, so that sintering of Rh is suppressed by this Ba. In addition, this Ba forms a compound with P in the exhaust gas, and suppresses performance degradation due to P poisoning of the catalyst. That is, Ba produces barium phosphate together with P, but this compound does not hinder gas diffusion and therefore does not degrade catalyst performance. Thus, when Ba combines with P, the amount of P poisoning of the catalyst is relatively reduced, which is advantageous for maintaining the activity of the catalyst.
[0012]
Furthermore, although Pd is advantageous for HC purification at low temperatures, it is known that Pd is likely to be thermally deteriorated and to be easily poisoned by lead, sulfur and the like. On the other hand, in the present invention, this Pd is disposed in the inner catalyst layer together with the oxygen storage material, and is covered with the outer catalyst layer, so that Pd thermal deterioration and poisoning are avoided by the barrier effect of the outer catalyst layer. The low temperature activity of the catalyst can be improved. Then, since the component in which Rh and Ba are supported on the Ce / Zr / Nd composite oxide is arranged in the outer catalyst layer, which is more severe in terms of heat and poisoning than the inner catalyst layer, Thus, even if the catalyst is exposed to high temperature, the decrease in oxygen storage capacity and Rh sintering are suppressed, and P poisoning is also suppressed, which is advantageous in maintaining the catalyst performance for a long period of time.
[0013]
Although Ba is effective for suppressing Rh sintering and P poisoning even in a small amount, if the amount is too small, the effect is reduced accordingly. On the other hand, when the amount of Ba supported is too large, the activity is reduced by covering the noble metal component of the catalyst such as Rh. In the case of a ceramic support, Ba attacks the support to make it brittle, and the catalyst coating layer easily peels off from the support. For this reason, it is preferable that the Ba loading is about 5 to 30 g / L, and more preferably 10 to 22 g / L or 10 to 15 g / L.
[0014]
The invention according to claim 2 is the exhaust gas purifying catalyst according to claim 1,
The catalyst layer located outside the inner catalyst layer further contains alumina supporting Pt .
[0015]
That is, in the catalyst layer positioned outside, comprising a Ce · Zr · Nd composite oxide carrying Rh, from containing alumina carrying Pt, HC, the advantageous for efficiently purifying CO and NOx .
[0016]
【The invention's effect】
As described above, according to the first aspect of the present invention, the catalyst layer located on the inner side contains the component in which Pd is supported on alumina, the oxygen storage material, and Ba. The catalyst layer located on the outer side contains a component in which Rh and Ba are supported on a Ce / Zr / Nd composite oxide, and the amount of Ba supported per liter of the support is 5 to 30 g. It is possible to improve the low-temperature activity of the catalyst while avoiding poisoning, and to reduce the oxygen storage capacity when the catalyst is exposed to a high temperature or Rh without lowering the activity of Rh or deteriorating the carrier. Sintering can be suppressed, and P poisoning can also be suppressed, which is advantageous in maintaining catalyst performance for a long period of time.
[0017]
According to the second aspect of the present invention, in the exhaust gas purifying catalyst according to the first aspect, the catalyst layer located on the outer side supports the Ce / Zr / Nd composite oxide supporting Rh and Pt. Therefore, it is advantageous for efficiently purifying HC, CO and NOx.
[0018]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0019]
In FIG. 1, 1 is a spark ignition engine of an automobile, 2 is its cylinder, 4 is its combustion chamber, 5 is a spark plug, 6 is an intake passage, and 7 is an exhaust passage. A three-way catalyst 8 is disposed in the exhaust passage 7.
[0020]
<Configuration of three-way catalyst>
As shown in FIG. 2, the three-way catalyst 8 is formed by forming an inner catalyst layer 12 and an outer catalyst layer 13 in a layered manner on the surface of the carrier 11.
[0021]
The carrier 11 is a cordierite honeycomb carrier. The inner catalyst layer 12 contains a Pd / alumina component obtained by supporting Pd on alumina, cerium oxide (ceria), a Ce.Zr.Nd composite oxide, and a binder. The outer catalyst layer 13 contains a Pt / alumina component in which Pt is supported on alumina, an Rh / Ce / Zr / Nd component in which Rh is supported on a Ce / Zr / Nd composite oxide, and a binder. .
[0022]
Further, Ba is supported on the inner catalyst layer 12 and the outer catalyst layer 13 by an impregnation method. Therefore, the Rh / Ce · Zr · Nd component of the outer catalyst layer 13 is such that Rh and Ba are supported on the Ce · Zr · Nd composite oxide. That is, the component is (Rh + Ba) / Ce · Zr · Nd.
[0023]
Both the cerium oxide and the Ce · Zr · Nd composite oxide function as an oxygen storage material. A small amount (5%) of La is added to the alumina in order to suppress thermal degradation. As the binder, zirconia is used to improve heat resistance.
[0024]
<Method for producing three-way catalyst 8>
Next, a method for producing the three-way catalyst 8 will be described.
[0025]
-Formation of inner catalyst coat layer-
Pd / alumina catalyst powder is obtained by dropping a palladium nitrate aqueous solution into activated alumina powder to which 5% by mass of La is added and drying and firing at 500 ° C.
[0026]
The catalyst powder, cerium oxide, Ce · Zr · Nd composite oxide and binder (zirconyl nitrate) are mixed, water and nitric acid are added thereto, and mixed and stirred with a disperser to obtain a slurry. By immersing the cordierite honeycomb carrier 11 in this slurry and pulling it up and removing excess slurry by air blow, a predetermined amount of slurry is coated on the carrier. Thereafter, the honeycomb carrier is heated at a constant heating rate from room temperature to 500 ° C. over a period of 1.5 hours, and held at that temperature for 2 hours (drying / firing), whereby the inner catalyst coating layer. Form.
[0027]
-Formation of outer catalyst coat layer-
An aqueous solution of dinitrodiamine platinum nitrate is added dropwise to activated alumina powder to which 5% by mass of La is added, and dried and calcined at 500 ° C. to obtain Pt / alumina catalyst powder. In addition, a rhodium nitrate aqueous solution is dropped onto the Ce · Zr · Nd composite oxide, and dried and fired at 500 ° C. to obtain Rh / Ce · Zr · Nd catalyst powder.
[0028]
The Pt / alumina catalyst powder, Rh / Ce · Zr · Nd catalyst powder and binder (zirconyl nitrate) are mixed, water and nitric acid are added thereto, and the mixture is stirred with a disperser to obtain a slurry. The carrier is coated with a predetermined amount of slurry by immersing the cordierite honeycomb carrier 11 on which the inner catalyst coating layer is formed in this slurry, and repeating the operation of pulling up and removing excess slurry by air blow. Thereafter, the honeycomb carrier is heated at a constant heating rate from room temperature to 500 ° C. over a period of 1.5 hours and held at that temperature for 2 hours (drying / firing), whereby the outer catalyst coating layer. Form.
[0029]
-Ba impregnation support-
The inner catalyst coat layer and the outer catalyst coat layer formed on the honeycomb carrier 11 as described above are impregnated with an aqueous solution of barium acetate. Thereafter, the honeycomb carrier is heated from room temperature to 200 ° C. at a substantially constant heating rate over 1.5 hours, held at that temperature for 2 hours (dried), and substantially until 200 ° C. to 500 ° C. The temperature is raised over a period of 4 hours at a constant rate of temperature rise and held at that temperature for 2 hours (firing).
[0030]
<Example>
A three-way catalyst having the following constitution was obtained by the above production method.
[0031]
-Inner catalyst layer-
Pd / alumina component loading: 63.529 g / L
(Pd loading: 4.091 g / L)
Amount of cerium oxide supported: 4.770 g / L
Ce / Zr / Nd composite oxide supported amount: 4.770 g / L
Zirconia binder loading: 7.120 g / L
-Outer catalyst layer-
Pt / alumina component loading amount: 25.583 g / L
(Pt loading amount: 0.136 g / L)
Rh / Ce · Zr · Nd component loading amount: 56.119 g / L
(Rh loading: 0.273 g / L)
Amount of supported zirconia binder; 9.08 g / L
-Ba-
Ba supported amount: 11.00 g / L
Note that the supported amount is an amount per 1 L of the honeycomb carrier.
[0032]
<Comparative example>
A comparative catalyst was prepared under the same conditions and method as in the above example except that cerium oxide was used instead of the Ce / Zr / Nd composite oxide.
[0033]
<Evaluation test>
After attaching the catalyst of the said Example and comparative example to the exhaust pipe of the engine and performing bench aging, the discharge | emission amount of each of HC, CO, and NOx in FTP test mode (total) was measured. Bench aging is a 20-hour cycle in which a low-load operation where the exhaust gas temperature at the catalyst inlet is 660 ° C and a high-load operation where the temperature is 880 ° C are alternately repeated for 1 minute, while P is added. The engine oil is continuously supplied to the intake manifold by a pump. The catalyst was given a P equivalent to tens of thousands of kilometers.
[0034]
The result is shown in FIG. The Ce · Zr · Nd composite oxide in the figure relates to the example, and the cerium oxide relates to the comparative example. In addition, the amount of HC emission has expanded 10 times. According to the figure, all of HC, CO, and NOx are less in the example than in the comparative example. This is recognized to be due to the effect of the Ce · Zr · Nd composite oxide.
[0035]
That is, in the case of the example, the Ce · Zr · Nd composite oxide has higher heat resistance than cerium oxide, and thus maintains a high oxygen storage capacity even after bench aging. As a result, this Ce · Zr · It is considered that the decrease in the activity of Rh supported on the Nd composite oxide is small. In addition, Ba suppresses Rh sintering and reduces the P poisoning amount of the catalyst. For this reason, in the Example, it is thought that discharge | emission amount of HC, CO, and NOx after bench aging is smaller than a comparative example.
[0036]
<About Ba loading>
-Test 1
Regarding the above examples, the catalysts having Ba loadings of 0.72 g / L, 11.1 g / L, 14.5 g / L, and 19.3 g / L were prepared, and the same bench aging was performed for each catalyst. After the application, emissions of HC and NOx in the FTP test mode (total) were measured. The result is shown in FIG.
[0037]
According to the figure, the magnitude of the amount of Ba supported does not significantly affect the HC emission amount (it has been confirmed that it does not significantly affect the CO emission amount), but is large in the NOx emission amount. It has an influence. Specifically, the NOx emission amount is the smallest when the Ba carrying amount is about 10 to 15 g / L, and the NOx emission amount increases both when the Ba carrying amount becomes smaller and when the Ba carrying amount becomes larger. ing.
[0038]
The reason why the NOx emission amount decreases as the Ba loading amount increases in the range of the Ba loading amount of 15 g / L or less is considered to be due to the effect of improving the P poisoning resistance of the catalyst by Ba. In contrast, when the amount of Ba supported exceeds 15 g / L, Ba covers the noble metal and other catalyst components to reduce its activity, or the honeycomb carrier 11 becomes brittle and the catalyst layer peels off. Therefore, it is considered that the amount of NOx emissions is increasing.
[0039]
-Test 2-
Regarding the above examples, catalysts having Ba loadings of 0 g / L, 11 g / L, 22 g / L, and 30 g / L were prepared, and after engine aging was performed for each, HC, CO, and NOx were measured in a rig test. T50, C400 and C500 related to purification were measured.
[0040]
In engine aging, a catalyst is attached to the exhaust pipe of an engine, the engine is operated for 100 hours so that the temperature of the catalyst becomes 900 ° C., and engine oil with P added is supplied to the intake manifold by a pump during the operation period. Is to continue.
[0041]
The rig test was performed by removing the engine-aged catalyst from the exhaust pipe and cutting it into a cylindrical shape having a diameter of 2.54 cm and a length of 5 cm, and attaching this to a fixed bed flow type reaction evaluation apparatus. The simulated exhaust 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 composition of the mainstream gas with A / F = 14.7 is as follows. The amount of simulated exhaust gas flowing into the catalyst was 25 L / min.
(Mainstream gas)
CO ₂ ; 13.9%, O ₂ ; 0.6%, CO; 0.6%, H ₂ ; 0.2%,
C ₃ H ₆ ; 0.056%, NO; 0.1%, H ₂ O; 10%, remaining N ₂
As the above-mentioned variable gas, O ₂ is used when the A / F is moved to the lean side (A / F = 15.6), and when the A / F is moved to the rich side (A / F = 13.9). H ₂ and CO were used.
[0042]
T50 gradually increases the simulated exhaust gas temperature, and when the concentration of the component (HC, CO or NOx) of the gas detected downstream of the catalyst becomes half of the component concentration of the gas flowing into the catalyst ( This is the catalyst inlet gas temperature (light-off temperature) when the purification rate reaches 50%.
[0043]
C400 is the purification rate of each component when the simulated exhaust gas temperature at the catalyst inlet is 400 ° C., and C500 is the purification rate of each component when the simulated exhaust gas temperature at the catalyst inlet is 500 ° C. is there.
[0044]
The result of T50 is shown in FIG. Under severer endurance conditions, T50 decreases as the Ba loading increases for all of HC, CO, and NOx until the Ba loading reaches 22 g / L. That is, the low temperature activity is improved. This is considered to be due to the suppression of P poisoning by Ba and the sintering suppression effect of Rh. When the amount of Ba supported was 30 g / L, T50 was slightly deteriorated (increased) because Ba covered the catalytic metals (Pd, Pt, Rh) and slightly decreased their activity. It is done.
[0045]
The result of C400 is shown in FIG. 6, and the result of C500 is shown in FIG. As for C400 and C500, as in T50, the purification rate increases as the amount of Ba supported increases for all of HC, CO, and NOx (high-temperature activity improves), but the amount of Ba supported increases. When it becomes 30 g / L, it has fallen a little. The reason is considered to be the same as in the case of T50.
[0046]
In the case of Test 1, when the Ba loading is 19.3 g / L, the NOx purification performance is lower than when it is 11.1 g / L, whereas in the case of Test 2, the Ba loading is The purification performance of HC, CO, and NOx is better when the value is 22 g / L than when the value is 11 g / L. This is presumably because the aging conditions in Test 2 are stricter than in Test 1.
[0047]
That is, the addition of Ba has a problem that the catalytic metal is covered to lower its activity, and the honeycomb carrier becomes brittle and the catalyst layer is liable to be peeled off. However, when the aging conditions are severe, P poisoning and Rh Therefore, a larger amount of Ba supported is advantageous in terms of exhaust gas purification performance.
[0048]
From the results of Tests 1 and 2, it can be said that the Ba loading is preferably about 5 to 30 g / L so as not to be too small and not too large. Further, according to the result of Test 1, it can be said that the Ba loading is preferably 10 to 15 g / L. However, if there are strong concerns about P poisoning and Rh sintering, From the result of 2, it can be said that it is preferable to increase the amount of Ba supported to about 22 g / L.
[Brief description of the drawings]
FIG. 1 is a diagram showing an exhaust gas purifying device for an engine according to an embodiment of the present invention.
FIG. 2 is a sectional view showing a catalyst structure according to the embodiment.
FIG. 3 is a graph showing the exhaust gas purification performance after bench aging of an example of the present invention and a comparative example.
FIG. 4 is a graph showing the relationship between the Ba carrying amount and the HC and NOx emission amounts in an example of the present invention.
FIG. 5 is a graph showing the relationship between the amount of Ba supported and T50 related to purification of HC, CO, and NOx in an example of the present invention.
FIG. 6 is a graph showing the relationship between the amount of Ba supported and C400 of HC, CO, and NOx in an example of the present invention.
FIG. 7 is a graph showing the relationship between the amount of Ba supported and C500 of HC, CO, and NOx in an example of the present invention.
[Explanation of symbols]
1 Engine 7 Exhaust passage 8 Three-way catalyst 11 Support 12 Inner catalyst layer 13 Outer catalyst layer

Claims (2)

In the exhaust gas purifying catalyst containing Rh and Ba,
A plurality of catalyst layers are formed in layers on the support,
The catalyst layer located inside of the plurality of catalyst layers contains a component in which Pd is supported on alumina, an oxygen storage material, and Ba,
The catalyst layer located outside the inner catalyst layer contains a component in which the Rh and Ba are supported on a Ce / Zr / Nd composite oxide ,
An exhaust gas purifying catalyst, wherein the amount of Ba supported per liter of the carrier is 5 to 30 g . In the exhaust gas purifying catalyst according to claim 1,
The catalyst for exhaust gas purification , wherein the catalyst layer located outside the inner catalyst layer further contains alumina carrying Pt .

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