JP3826476B2 - Exhaust gas purification catalyst and method for producing the same - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust gas purifying catalyst and a method for producing the same, and in particular, it can simultaneously remove HC, CO and NOx in exhaust gas discharged from an automobile engine, at low and high temperatures. The present invention relates to an exhaust gas purifying catalyst having excellent purification performance and heat resistance, and a method for producing the same.
[0002]
[Prior art]
In general, exhaust gas exhausted from automobile engines contains air pollutants such as HC (hydrocarbon), CO (carbon monoxide), and NOx (nitrogen oxide). Regulations are being tightened globally. In particular, in California, where air pollution has become a major social problem, the corporate average NMOG regulations have been introduced, and the level of regulations on exhaust gases is gradually being strengthened. To achieve this, it is necessary to introduce low-pollution vehicles such as LEV or ULEV to the market (introduced in stages from 1997 to 2000).
[0003]
Thus, an exhaust system of an automobile engine is usually provided with an exhaust gas purification device (catalytic converter) using an exhaust gas purification catalyst in order to purify the exhaust gas. As such an exhaust gas purifying catalyst, a catalyst in which a noble metal catalyst such as platinum or rhodium is supported on a porous catalyst substrate such as alumina has been widely used.
[0004]
By the way, the exhaust gas purifying catalyst using the noble metal catalyst such as platinum or rhodium has a problem that the catalytic activity at low temperature is low. For this reason, when the exhaust gas temperature is low, for example, immediately after starting the engine, the temperature of the exhaust gas purifying catalyst is not sufficiently increased, and the emission performance of the engine is deteriorated. Therefore, in recent years, exhaust gas purifying catalysts using palladium having a high catalytic activity at low temperatures as a catalyst component have been proposed (for example, Japanese Patent Laid-Open Nos. 5-1884876, 4-72577 and 1). No. -281144).
[0005]
[Problems to be solved by the invention]
However, the conventional exhaust gas purification catalyst using palladium as a catalyst component has a problem that the NOx purification performance at a high temperature is low although the catalytic activity at a low temperature is high. In particular, in recent years, in an automobile engine, an exhaust gas purification device tends to be disposed in an upstream portion of an exhaust passage in order to quickly increase the temperature of the exhaust gas purification catalyst after the start of the engine. If the exhaust gas purification device is arranged in the upstream portion of the exhaust passage in this way, the temperature of the exhaust gas purification catalyst becomes high at normal times, so that there arises a problem that the NOx purification performance at a particularly high temperature is further lowered. .
[0006]
The present invention has been made to solve the above-described conventional problems, and while improving exhaust gas purification performance at low temperatures such as immediately after the start of engine start, without deteriorating the overall exhaust gas purification performance, In particular, it is an object or object to be solved to obtain an exhaust gas purification catalyst capable of enhancing NOx purification performance at high temperatures and a method for producing the same.
[0007]
[Means for Solving the Problems]
Specifically, a first aspect of the present invention, which has been made to solve the above problems, is a catalyst for purifying exhaust gas carrying palladium, wherein cerium oxide (CeO) is used. ₂ ) And a complex oxide of cerium (Ce) and praseodymium (Pr) (hereinafter referred to as “(Ce, Pr) complex oxide” or “(Ce, Pr) O”). ₂ ")").
[0008]
In this exhaust gas purification catalyst, palladium having high low-temperature activity is used as a catalyst component, so that the exhaust gas purification performance at a low temperature such as immediately after the start of the engine is enhanced. Since cerium oxide and (Ce, Pr) composite oxide enhance the catalytic activity of palladium at high temperatures, exhaust gas purification performance, particularly NOx purification performance at high temperatures can be enhanced. Moreover, the said effect can be exhibited, improving high temperature heat resistance.
In the present specification, the term “exhaust gas purification performance” simply refers to the comprehensive purification performance of HC, CO, and NOx.
[0009]
In the exhaust gas purification catalyst, the weight ratio of cerium oxide and (Ce, Pr) composite oxide is preferably 9/1 (9: 1) or less. In this case, the NOx purification performance of the exhaust gas purification catalyst at a high temperature can be further enhanced.
[0010]
In the exhaust gas purifying catalyst according to the first aspect, the weight ratio of cerium oxide to (Ce, Pr) composite oxide is preferably 7/3 (7: 3) or more. In this way, the CO purification performance at high temperatures of the exhaust gas purification catalyst is particularly enhanced.
[0011]
Furthermore, in the exhaust gas purifying catalyst according to the first aspect, the weight ratio of cerium oxide to (Ce, Pr) composite oxide is 9/1 (9: 1) to 7/3 (7: 3). It is particularly preferred that it is within the range. In this case, the NOx purification performance and CO purification performance at high temperatures of the exhaust gas purification catalyst are particularly enhanced, and the exhaust gas purification performance at low temperatures is further enhanced.
[0012]
In each of the above exhaust gas purifying catalysts, the ratio of praseodymium to cerium in the (Ce, Pr) composite oxide is preferably in the range of 3 to 50 mol%. In this way, the NOx purification rate of the exhaust gas purification catalyst is particularly increased.
The ratio is more preferably in the range of 3 to 20 mol%. In this way, the exhaust gas purification performance (for example, T50 temperature) at a low temperature for HC and CO is further enhanced, and the NOx purification rate is further enhanced.
The ratio is more preferably in the range of 5 to 20 mol%. In this way, the NOx purification rate can be further increased.
Furthermore, the ratio is more preferably in the range of 5 to 7 mol%. In this way, the exhaust gas purification performance (for example, T50 temperature) at a low temperature for HC, CO, and NOx can be further enhanced.
[0013]
In the exhaust gas purifying catalyst, instead of the (Ce, Pr) composite oxide, a composite oxide of cerium and terbium (Tb) (hereinafter referred to as “(Ce, Tb) composite oxide” or “( Ce, Tb) O ₂ May be used). Also in this case, substantially the same operation and effect as in the case of the exhaust gas purification catalyst using the (Ce, Pr) composite oxide can be obtained.
[0014]
In the exhaust gas purifying catalyst, the impurity content is preferably less than 1% by weight. In this case, the exhaust gas purification performance of the exhaust gas purification catalyst is further enhanced.
[0015]
According to a second aspect of the present invention, in the method for producing an exhaust gas purifying catalyst carrying palladium, a step of carrying palladium on a porous substrate, the substrate carrying palladium, and an oxidation It includes a step of producing a slurry containing cerium and a complex oxide of cerium and praseodymium as a solid content, and a step of firing the slurry to form an exhaust gas purification catalyst.
[0016]
The third aspect of the present invention is a method for producing an exhaust gas purifying catalyst carrying palladium, the step of mixing a porous substrate and cerium oxide to form a substrate mixture, A step of supporting palladium on the material mixture, a step of forming a slurry containing the above-mentioned base material mixture supporting palladium and a composite oxide of cerium and praseodymium as solids, and firing the slurry to purify exhaust gas And a step of forming a catalyst for use.
[0017]
According to the production method of the second or third aspect of the present invention, an exhaust gas purifying catalyst containing cerium oxide and a (Ce, Pr) composite oxide and containing palladium as a catalyst component can be easily obtained. In the exhaust gas purification catalyst manufactured in this way, the exhaust gas purification performance at low temperatures such as immediately after the start of the engine is enhanced by palladium contained as a catalyst component, while cerium oxide and (Ce, With the Pr) composite oxide, exhaust gas purification performance, particularly NOx purification performance at high temperatures can be enhanced.
[0018]
In each of the above production methods, it is preferable that the (Ce, Pr) composite mixture is prepared by coprecipitation of a cerium nitrate aqueous solution and a praseodymium nitrate aqueous solution. In this case, adjustment or setting of the ratio of cerium and praseodymium in the (Ce, Pr) composite oxide becomes easy.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be specifically described.
As shown in FIG. 2, an exhaust gas purification device 2 is interposed in an exhaust system 1 for exhausting exhaust gas of an automobile engine (not shown) to the atmosphere. In the exhaust gas purification device 2, Air pollutants such as HC, CO, NOx in the exhaust gas are H ₂ O, CO ₂ , N ₂ An exhaust gas purifying catalyst 3 for purifying by being decomposed into, for example, is packed. Further, the engine sets the air-fuel ratio to be richer than the theoretical air-fuel ratio (A / F = 14.7) (A / F = 13 to 14) for a predetermined period immediately after starting the engine, and after the predetermined period has elapsed, To set the air-fuel ratio to the stoichiometric air-fuel ratio, O ₂ Execute feedback control.
[0020]
Here, the exhaust gas purification device 2 is interposed in an upstream portion of the exhaust system 1, that is, an exhaust manifold. That is, the exhaust gas purification device 2 is a so-called direct connection type exhaust gas purification device. Therefore, the temperature of the exhaust gas introduced into the exhaust gas purification device 2 becomes relatively high, and the temperature rise of the exhaust gas purification catalyst 3 is promoted immediately after the start of the engine or the like, and the exhaust gas purification performance is enhanced. On the other hand, since the temperature of the exhaust gas purification catalyst during normal operation tends to increase, it is necessary to improve the exhaust gas purification performance at high temperatures.
[0021]
As shown in FIG. 1, in the exhaust gas purifying catalyst 3, a catalyst carrier layer 5 is formed (fixed) on a honeycomb carrier 4 made of cordierite, which is a carrier material having excellent heat resistance. Yes. Here, cordierite is used as the carrier material, but the carrier material is not limited to cordierite.
In this exhaust gas purification catalyst 3, only a single catalyst support layer 5 is formed on the carrier 4, but another catalyst support is provided outside, inside, or on both sides of the catalyst support layer 5. A layer may be formed.
[0022]
The catalyst support layer 5 basically includes palladium as a catalyst component (active species) and porous γ-alumina (γ-Al as a base material). ₂ O _Three ). The catalyst support layer 5 contains cerium oxide (ceria) and (Ce, Pr) composite oxide as promoters (OSC: oxygen storage agent). In this (Ce, Pr) composite oxide, cerium and praseodymium have a chemical bonding relationship and are crystallized. Here, the ratio of cerium and praseodymium can be set arbitrarily. That is, the chemical formula of the (Ce, Pr) composite oxide is expressed as (Ce _1-x , Pr _x ) O ₂ Then, the value of x can be arbitrarily set within the range of 0-1. Note that the cerium oxide and the (Ce, Pr) composite oxide are only physically mixed, and there is no chemical bonding relationship between them.
[0023]
Hereinafter, a method for producing the exhaust gas purifying catalyst 3 will be described. That is, the exhaust gas purification catalyst 3 is manufactured through the following steps.
(1) Heat treatment of γ-alumina
Pure γ-alumina powder is exposed to air at 900 ° C. for 50 hours to heat-treat the γ-alumina. Thereby, the thermal stability of γ-alumina is improved.
[0024]
(2) Loading of palladium
After the heat-treated γ-alumina and cerium oxide powder are physically mixed, an aqueous solution of a palladium compound (eg, dinitrodiamine palladium) as an active species is dropped into the mixture and impregnated, and then the mixture is dried. Let Thus, a palladium-supported powder in which palladium as a catalyst component (active species) is supported by γ-alumina and cerium oxide is obtained.
[0025]
(3) Synthesis of (Ce, Pr) composite oxide
After mixing a cerium nitrate aqueous solution and a praseodymium nitrate aqueous solution, ammonia is added to the mixture to cause coprecipitation. The precipitate is washed, further dried, and then fired at a temperature of about 600 ° C. Thereby, (Ce, Pr) composite oxide powder is obtained. The molar ratio of cerium to praseodymium in the (Ce, Pr) composite oxide is preferably set within the range of (99.9 to 70) :( 0.1 to 30).
[0026]
(4) Preparation of slurry for washcoat
The palladium-supported powder, the (Ce, Pr) composite oxide powder, water and a binder (for example, hydrated alumina) are mixed to prepare a slurry for washcoat. In this wash coat slurry, the weight ratio of cerium oxide to (Ce, Pr) composite oxide is within a predetermined range, for example, within a range of 9/1 (9: 1) to 7/3 (7: 3). Set to
[0027]
(5) Wash coat on carrier
A honeycomb-shaped carrier made of cordierite is dipped in the washcoat slurry, and then excess slurry is blown away. Thus, the carrier is coated with the washcoat slurry. The carrier thus coated is dried at a temperature of about 150 ° C., and then fired at a temperature of 500 ° C. for about 2 hours. Thereby, the exhaust gas purifying catalyst is completed. In this exhaust gas purifying catalyst, the carrying amount with respect to the carrier is set to 33 to 40% of the carrier weight.
[0028]
Hereinafter, the results of measuring the purification performance of the exhaust gas purifying catalyst according to the present invention (the present plan) thus manufactured are compared with the exhaust gas purifying catalyst (comparative example) manufactured by the conventional manufacturing method for comparison. This will be explained in contrast to that of).
The exhaust gas purifying catalyst as a comparative example was manufactured by the same manufacturing method as that of the exhaust gas purifying catalyst according to the present invention using only cerium oxide as a co-catalyst. The weight of cerium oxide is the same as in the case of the above-described embodiment (present plan).
[0029]
The evaluation (activity evaluation) of the exhaust gas purification performance of these both exhaust gas purification catalysts was performed under the following conditions using an atmospheric pressure fixed bed flow type reactor.
That is, as an exhaust gas to be purified, a model gas having the following composition equivalent to an exhaust gas corresponding to an air-fuel mixture having an air-fuel ratio A / F of 14.7 ± 0.9 (1 Hz) is used. It was. The space velocity of the model gas in the reactor is 60000 h. ^-1 It was.
<Composition of model gas>
Propylene: 1665ppmc H ₂ : 0.2%
NOx: 0.1% O ₂ : 0.6%
CO: 0.6% CO ₂ : 13.9%
N ₂ : Balance
[0030]
FIG. 3 shows measured values of T50 temperature for HC, CO and NOx after heat treatment of the exhaust gas purifying catalyst according to the present invention (the present plan) and a conventional exhaust gas purifying catalyst which is a comparative example. The heat treatment of both catalysts was performed by exposing them to air at 1000 ° C. for 24 hours in order to confirm the high temperature heat resistance.
Here, the T50 temperature is the exhaust gas inlet temperature [° C.] when the purification rate of HC, CO, or NOx is 50%. That is, the T50 temperature is an index for evaluating the exhaust gas purification performance or low temperature activity of the exhaust gas purification catalyst at low temperatures, and the lower the T50 temperature, the higher the exhaust gas purification performance or low temperature activity at low temperatures. become.
[0031]
FIG. 4 shows measured values of the NOx purification rate at 400 ° C. and 500 ° C. after heat treatment of the exhaust gas purification catalyst according to the present invention (the present plan) and a conventional exhaust gas purification catalyst as a comparative example. . The heat treatment of both catalysts was performed by exposing them to air at 1000 ° C. for 24 hours in order to confirm the high temperature heat resistance.
[0032]
As shown in FIG. 3, in the exhaust gas purification catalyst according to the present invention, the T50 temperature for CO and NOx is clearly lower than that of the conventional exhaust gas purification catalyst as a comparative example, and the HC The T50 temperatures for are approximately the same. Therefore, the exhaust gas purification catalyst according to the present invention is superior in exhaust gas purification performance or low temperature activity at a low temperature as compared with conventional exhaust gas purification catalysts.
[0033]
Further, as shown in FIG. 4, the exhaust gas purification catalyst according to the present invention has clearly higher NOx purification rates at 400 ° C. and 500 ° C. than the conventional exhaust gas purification catalyst as a comparative example. It has become. Therefore, the exhaust gas purification catalyst according to the present invention is superior in NOx purification performance at high temperatures as compared with conventional exhaust gas purification catalysts. That is, the exhaust gas purifying catalyst according to the present invention can achieve both the exhaust gas purifying performance at a low temperature and the NOx purifying performance at a high temperature.
In this exhaust gas purification catalyst, the content of impurities is preferably less than 1% by weight. In this case, the exhaust gas purification performance of the exhaust gas purification catalyst is further enhanced.
[0034]
In the exhaust gas purifying catalyst according to the present invention (hereinafter referred to as “the present exhaust gas purifying catalyst”), (Ce, Pr) composite oxide is used as a promoter. However, in the composite oxide, even when the praseodymium is replaced with another rare earth element (X) other than praseodymium, the same action and effect may be obtained. Therefore, various exhaust gas purifying catalysts using cerium and a complex oxide of such rare earth elements (hereinafter referred to as “(Ce, X) complex oxide”) as a co-catalyst are prepared, and the exhaust gas purifying catalyst. The exhaust gas purification performance of was investigated.
[0035]
5 to 7, as rare earth elements (X), yttrium (Y), lanthanum (La), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), HC, NOx of an exhaust gas purifying catalyst containing (Ce, X) composite oxide when terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er) or ytterbium (Yb) is used The measured value of T50 temperature and C400 temperature about CO and CO is shown. 5 to 7, for comparison, HC, NOx, and CO of two conventional exhaust gas purifying catalysts (Comparative Example A and Comparative Example B) that do not contain such a complex oxide are shown. The measured values of T50 temperature and C400 purification rate are also shown.
The properties of the exhaust gas purifying catalyst containing the (Ce, X) composite oxide and Comparative Examples A and B are as follows.
That is, the exhaust gas purifying catalyst of the present invention contains (Ce, Pr) composite oxide, cerium oxide, palladium and alumina as described above, but the exhaust gas purifying catalyst containing (Ce, X) composite oxide. Is obtained by replacing the (Ce, Pr) composite oxide in the present exhaust gas purification catalyst with the same amount of (Ce, X) composite oxide. Therefore, the present exhaust gas purification catalyst and the exhaust gas purification catalyst containing the (Ce, X) composite oxide have the same amount of composite oxide (although different types of rare earth elements), cerium oxide, palladium and alumina. Will be included.
In Comparative Examples A and B, both of the exhaust gas purifying catalyst containing (Ce, X) composite oxide are replaced with cerium, that is, (Ce, Pr) in the present exhaust gas purifying catalyst. The composite oxide is mixed with an amount of cerium oxide (CeO) corresponding to the composite oxide. ₂ ). However, the surface area of cerium oxide replaced with the (Ce, Pr) composite oxide is 100 m in Comparative Example A. ² / G, 70 m in Comparative Example B ² / G.
[0036]
Here, the C400 purification rate is the purification rate [%] of HC, CO, or NOx when the exhaust gas inlet temperature is 400 ° C. The purification rate when the exhaust gas inlet temperature is 500 ° C. is called the C500 purification rate. That is, the C400 purification rate is an index for evaluating the exhaust gas purification performance at a high temperature of about 400 ° C., and the C500 purification rate is an index for evaluating the exhaust gas purification performance at a high temperature of about 500 ° C.
[0037]
In each of these exhaust gas purification catalysts, the amount of palladium supported is 1.3 g / L-cat, and γ-alumina is used as the base material. In the exhaust gas purifying catalyst containing these composite oxides, the molar ratio of cerium to the rare earth element is 7/3. That is, if the element symbol of each rare earth element is represented by X, each composite oxide can be expressed as (Ce _0.7 , X _0.3 ) O ₂ It is expressed. Each exhaust gas purification catalyst was heat-treated by exposing the exhaust gas purification catalyst to air at 1000 ° C. for 24 hours. As the exhaust gas to be processed, a model gas equivalent to an exhaust gas corresponding to an air-fuel mixture having an air-fuel ratio A / F of 14.7 ± 0.9 is used.
[0038]
As shown in FIG. 5 to FIG. 7, among various composite oxides, those using praseodymium as a rare earth element are considered to have the best exhaust gas purification performance as a whole. However, even when terbium is used as the rare earth element, the overall exhaust gas purification performance, particularly NOx and CO purification performance at high temperatures, is quite excellent, so (Ce, Tb) composite oxides are also used for exhaust gas purification. It is considered that it can be used as a co-catalyst for the catalyst.
[0039]
Hereinafter, the results of various experiments conducted to determine the preferred ratio of praseodymium to cerium in the (Ce, Pr) composite oxide used as the promoter for the exhaust gas purifying catalyst will be described.
In the following, the ratio of praseodymium to cerium in the (Ce, Pr) composite oxide (hereinafter referred to as “Pr ratio”) is expressed as a molar ratio. That is, when this Pr ratio is represented by x, the (Ce, Pr) composite oxide is (Ce _1-x , Pr _x ) O ₂ It will be expressed.
[0040]
FIG. 8 shows the T50 temperature of HC, CO and NOx in several exhaust gas purification catalysts (after 1000 ° C. × 24 hr endurance) containing (Ce, Pr) composite oxides having different Pr ratios x. The actual measurement value is shown.
Further, FIG. 9 shows the C400 purification rate for CO and NOx in several exhaust gas purification catalysts (after 1000 ° C. × 24 hr durability) containing (Ce, Pr) composite oxides having different Pr ratios x. And the measured value () of C500 purification rate is shown.
Further, in FIGS. 10 and 11, C400 purification rates for CO and NOx in several exhaust gas purification catalysts containing (Ce, Pr) composite oxides having different Pr ratios x in FIG. 9 respectively. And the thing which represented the C500 purification rate with the bar graph is shown.
[0041]
According to FIG. 8, when the Pr ratio x is in the range of about 0.03 to 0.07, the T50 temperature of HC and CO is particularly low, and therefore the purification performance of HC and CO is particularly good particularly at low temperatures. It can be seen that it is.
According to FIG. 9, when the Pr ratio x is in the range of about 0.05 to 0.5, the NOx purification rate is high at high temperatures, and particularly when it is in the range of 0.07 to 0.2. It can be seen that the NOx purification rate is very high. It can also be seen that when the Pr ratio x is in the range of about 0.05 to 0.07, the CO purification rate at high temperatures is high.
Thus, according to the actual measurement values shown in FIGS. 8 to 11, the appropriate range of the Pr ratio x is approximately 0.03 to 0.5, and in this case, the NOx purification rate is increased particularly at high temperatures. However, the Pr ratio x is preferably in the range of 0.03 to 0.2. In this case, the NOx purification rate and the CO purification rate are increased particularly at high temperatures. The Pr ratio x is more preferably in the range of 0.05 to 0.2. In this case, the NOx purification rate and the CO purification rate at a high temperature are further increased. However, it is best that the Pr ratio x is in the range of 0.05 to 0.07. In this case, the NOx purification rate and the CO purification rate at a high temperature are greatly increased, and HC, CO at a low temperature are also obtained. And NOx purification performance is enhanced.
[0042]
By the way, in the exhaust gas purifying catalyst according to the present invention, as described above, cerium oxide and (Ce, Pr) composite oxide are used as a co-catalyst, and these are simply in a physical mixed state. Depending on the mixing ratio, the exhaust gas purification performance of the exhaust gas purification catalyst also changes. Hereinafter, in the exhaust gas purifying catalyst according to the present invention, results of various experiments conducted for obtaining a preferable mixing ratio of cerium oxide to (Ce, Pr) composite oxide will be described.
In FIG. 12 or FIG. 13, the mixing ratio of cerium oxide to the (Ce, Pr) composite oxide is expressed in terms of weight percent of cerium oxide in the entire cocatalyst (hereinafter this mixing ratio is referred to as “oxidation”). Called weight percent of cerium)). For example, if the weight percent of cerium oxide is 0, the promoter is composed of only (Ce, Pr) composite oxide, and if it is 50%, cerium oxide and (Ce, Pr) composite oxide. Is contained in the cocatalyst at equal weight, and if it is 100%, the cocatalyst is composed only of cerium oxide.
[0043]
14 or 15, the mixing ratio of cerium oxide to (Ce, Pr) composite oxide is the weight ratio of cerium oxide to (Ce, Pr) composite oxide in the promoter (CeO). ₂ / (Ce, Pr) O ₂ ). For example, this CeO ₂ / (Ce, Pr) O ₂ Is 0/10, the promoter is composed only of (Ce, Pr) composite oxide, and if it is 5/5, cerium oxide and (Ce, Pr) composite oxide are in the promoter. In the case of 10/0, the promoter is composed only of cerium oxide.
[0044]
FIG. 12 shows measured values of T50 temperatures for HC, CO, and NOx in several exhaust gas purifying catalysts including cocatalysts each having different weight percentages of cerium oxide.
FIG. 13 shows measured values of the C400 purification rate and the C500 purification rate for CO and NOx in several exhaust gas purification catalysts containing cocatalysts having different weight percentages of cerium oxide.
Further, FIGS. 14 and 15 respectively show CeO in FIG. ₂ / (Ce, Pr) O ₂ Shows bar graphs of C400 purification rate and C500 purification rate for CO and NOx in several exhaust gas purification catalysts including different promoters.
[0045]
According to the actual measurement values shown in FIGS. 12 to 15, the T50 temperatures of HC, CO, and NOx are generally lower as the weight percentage of cerium oxide is higher. When the weight percentage of cerium oxide exceeds 90%, the T50 temperature seems to rise again. Further, when the weight percent of cerium oxide is approximately 90% or less, the NOx C400 purification rate and C500 purification rate are high. Further, when the weight percent of cerium oxide is approximately 70% or more, the C400 purification rate and C500 purification rate of CO are high. Therefore, if the weight percent of cerium oxide is set within a range of approximately 70% to 90%, the exhaust gas purification catalyst can be improved in exhaust gas purification performance at low temperatures and NOx and CO purification performance at high temperatures. It is thought that it can be increased.
[Brief description of the drawings]
FIG. 1 is a longitudinal sectional view of an exhaust gas purifying catalyst according to the present invention.
2 is a longitudinal cross-sectional explanatory view of an exhaust gas purification apparatus using the exhaust gas purification catalyst shown in FIG.
FIG. 3 is a graph showing T50 temperatures of HC, CO, and NOx of an exhaust gas purifying catalyst according to the present invention in comparison with a comparative example.
FIG. 4 is a view showing NOx purification rates at 400 ° C. and 500 ° C. of an exhaust gas purification catalyst according to the present invention in comparison with a comparative example.
FIG. 5 is a graph showing T50 temperature and C400 purification rate for HC of various exhaust gas purification catalysts containing complex oxides of cerium and rare earth elements.
FIG. 6 is a graph showing T50 temperature and C400 purification rate for NOx of various exhaust gas purification catalysts containing complex oxides of cerium and rare earth elements.
FIG. 7 is a graph showing T50 temperature and C400 purification rate for CO of various exhaust gas purification catalysts containing complex oxides of cerium and rare earth elements.
FIG. 8 is a graph showing measured values of T50 temperatures for HC, CO, and NOx in several exhaust gas purifying catalysts containing (Ce, Pr) composite oxides having different Pr ratios x.
FIG. 9 is a graph showing measured values of C400 purification rate and C500 purification rate for CO and NOx in several exhaust gas purification catalysts containing (Ce, Pr) composite oxides having different Pr ratios x. .
FIG. 10 is a graph showing another measured value of C400 purification rate and C500 purification rate for CO in several exhaust gas purification catalysts containing (Ce, Pr) composite oxides each having a different Pr ratio x. is there.
FIG. 11 is a graph showing another measured value of C400 purification rate and C500 purification rate for NOx in several exhaust gas purification catalysts containing (Ce, Pr) composite oxides having different Pr ratios x. is there.
FIG. 12 is a graph showing measured values of T50 temperatures for HC, CO, and NOx in several exhaust gas purifying catalysts including cocatalysts each having different weight percentages of cerium oxide.
FIG. 13 is a graph showing measured values of C400 purification rate and C500 purification rate for CO and NOx in several exhaust gas purification catalysts containing cocatalysts with different weight percentages of cerium oxide.
FIG. 14 Weight ratio CeO ₂ / (Ce, Pr) O ₂ 6 is a graph showing another measured value of the C400 purification rate and the C500 purification rate for CO in several exhaust gas purification catalysts including different promoters.
FIG. 15 Weight ratio CeO ₂ / (Ce, Pr) O ₂ 6 is a graph showing another measured value of the C400 purification rate and the C500 purification rate for NOx in several exhaust gas purification catalysts including different promoters.
[Explanation of symbols]
1 ... Exhaust system
2 ... Exhaust gas purification device
3 ... Exhaust gas purification catalyst
4 ... Carrier
5 ... Catalyst support layer

Claims (11)

In the exhaust gas purifying catalyst carrying palladium,
An exhaust gas purifying catalyst comprising cerium oxide and a composite oxide of cerium and praseodymium. The exhaust gas purification catalyst according to claim 1, wherein a weight ratio of the cerium oxide to the composite oxide is 9/1 or less. The exhaust gas purifying catalyst according to claim 1, wherein a weight ratio of the cerium oxide to the composite oxide is 7/3 or more. The exhaust gas purification catalyst according to claim 1, wherein a weight ratio of the cerium oxide and the composite oxide is in a range of 9/1 to 7/3. The exhaust gas purifying catalyst according to any one of claims 1 to 4, wherein a ratio of praseodymium to cerium in the composite oxide is in a range of 3 to 50 mol%. 6. The exhaust gas purifying catalyst according to claim 5, wherein the ratio of praseodymium to cerium in the composite oxide is in the range of 3 to 20 mol%. The exhaust gas purifying catalyst according to claim 6, wherein the ratio of praseodymium to cerium in the composite oxide is in the range of 5 to 20 mol%. 8. The exhaust gas purifying catalyst according to claim 7, wherein a ratio of praseodymium to cerium in the composite oxide is in a range of 5 to 7 mol%. A step of supporting palladium on a porous substrate;
Producing a slurry containing the above-mentioned substrate supporting palladium, cerium oxide, and a composite oxide of cerium and praseodymium as a solid content;
A process for producing an exhaust gas purification catalyst by firing the slurry to form an exhaust gas purification catalyst. A step of mixing a porous substrate and cerium oxide to form a substrate mixture;
A step of supporting palladium on the base material mixture;
Producing a slurry containing the above-mentioned base material mixture carrying palladium and a composite oxide of cerium and praseodymium as a solid content;
A process for producing an exhaust gas purification catalyst by firing the slurry to form an exhaust gas purification catalyst. 11. The exhaust gas purifying catalyst according to claim 9 or 10, wherein the composite mixture of cerium and praseodymium is prepared by coprecipitation of a cerium nitrate aqueous solution and a praseodymium nitrate aqueous solution. Manufacturing method.

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