JPS61222539A - Preparation of monolithic type exhaust gas purifying catalyst - Google Patents

Abstract

PURPOSE:To enhance the purifying action of a ternary catalyst, by forming the platinum group element fixed to activated alumina into an aqueous slurry along with a cerium hydroxide powder and applying said aqueous slurry to a monolithic carrier before additionally applying the other component. CONSTITUTION:At least one of compounds of platinum group elements are supported by an activated alumina powder in a dispersed state to fix at least one of platinum group metals onto activated alumina. This supported platinum group element composition is formed into an aqueous slurry along with a cerium hydroxide powder to prepare a slurry composition which is, in turn, applied to a monolithic carrier. On the other hand, a rhodium compound is supported by an activated alumina powder in a dispersed state to fix rhodium onto activated alumina while the obtained composition is formed into an aqueous slurry which is, in turn, additionally applied to the monolithic carrier, to which the aforementioned coating treatment was applied, to form an exhaust gas purifying catalyst.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a monolithic catalyst for exhaust gas purification. To explain in detail, the present invention deals with hydrocarbons (hereinafter referred to as HC), which are harmful components contained in exhaust gas.
, - relates to a method for producing a monolithic catalyst for removing carbon oxides (hereinafter referred to as CO) and nitrogen oxides (hereinafter referred to as NOx). More specifically, the present invention provides a method for reducing HC, Co, and NO in the exhaust gas of an internal combustion engine when it is operated near the stoichiometric point of air-to-fuel ratio.
x can be made stable and substantially harmless at the same time, and 800
The present invention relates to a method for manufacturing an integrally structured catalyst for exhaust gas purification that exhibits little deterioration even when exposed to high temperatures of .degree. C. or higher.

A catalyst for simultaneously removing three components of HC, Co and NOx in exhaust gas of an internal combustion engine with one catalytic converter,
So-called three-way catalysts are installed in some cars that comply with the 1953 regulations, and recently, the number of cars equipped with three-way catalysts has been increasing, partly as a measure to improve fuel efficiency. In this case, the catalyst is often installed under the floor, and in some cases it is installed directly under the manifold of other engines.

When an engine equipped with this three-way catalytic converter is operated near the stoichiometric air/fuel ratio (A/F), the
Exhaust gas is discharged with the components most effectively purified, but in order to make the three-way catalyst work even more effectively, an electronically controlled fuel injection device or venturi is used to supply fuel using an injection pump to maintain a constant rate of F/F. A method is adopted in which A/F is controlled using a vaporizer. However, depending on the control method, the A/F may vary over a wide range away from the stoichiometric equivalence point, and in the case of sudden changes in operation such as acceleration/deceleration, the integral structure catalyst In order to prevent melting due to a sudden rise in temperature, fuel supply may be partially or completely cut off and the fuel may be exposed to a lean atmosphere.

In other words, the three-way catalyst is not always exposed to ideal exhaust gas during A/F operation, and when the catalyst is exposed to high temperatures under such conditions, the components contained in the catalyst, especially rhodium, Platinum is susceptible to thermal degradation. Therefore, a wide range of A/
A three-way catalyst that exhibits stable purification performance even when operated at F and exhibits little deterioration is desired. If such a three-way catalyst could be obtained, it would be advantageous because it would be possible to reduce the amount of supported platinum metal elements while maintaining the same purification performance, and such a high-performance catalyst was desired. .

Conventionally, in order to meet this demand, it has been widely known that cerium oxide is used in coexistence with a platinum metal element as a substance with oxygen storage ability. A method of supporting activated alumina in the form of a salt such as cerium nitrate has been used. However, the method of supporting activated alumina with cerium in the form of a water-soluble salt such as cerium nitrate has the drawback of poor durability, particularly poor heat resistance.

Further, JP-A-58-122044 describes a method in which the catalyst component raw material is used in the form of carbonate, for example, lanthanum in the form of lanthanum carbonate. However, when this lanthanum carbonate is made to coexist with a small amount of inorganic or organic acid when a catalyst composition consisting of activated alumina or a catalytically active substance is pulverized into a slurry in an aqueous medium, the carbonate radicals react with these acid radicals. As a coating composition, the slurry has a strong chicken-like character, which makes it difficult to coat smoothly.

An object of the present invention is to provide a method that overcomes these drawbacks of the conventional methods. As a result of studies conducted by the present inventors to achieve this objective, the use of cerium hydroxide powder as a cerium raw material dramatically improves heat resistance and durability compared to conventional catalysts using cerium nitrate, and shows low-temperature activity. In particular, using fine particles of a certain particle size within a certain water content range as cerium hydroxide allows for better control of dispersibility, and adding a small amount of acid during the production of a coating liquid to create a slurry. It has been found that it is possible to produce a good dispersion liquid even when pulverized into a fine powder.

The present invention can be specified as follows.

(1) A compound of at least one platinum group element is dispersed and supported on activated alumina powder, the platinum group element is fixed on the activated alumina, and then this is further formed into an aqueous slurry with cerium hydroxide powder. A slurry composition is coated on a monolithic carrier, a rhodium compound is dispersed and supported on activated alumina powder, and rhodium is fixed on the activated alumina. A method for producing an integrated structure exhaust gas purifying catalyst characterized by additionally coating a carrier.

(2) At least one platinum group element compound is dispersed and supported on activated alumina powder, the platinum group element is fixed on the activated alumina, and this is further formed into an aqueous slurry with cerium hydroxide powder, resulting in a slurry composition. A slurry is coated on a monolithic structure carrier, then a rhodium compound and a zirconium compound are dispersed and supported on activated alumina powder, rhodium and zirconia are fixed on the activated alumina, and this is made into an aqueous slurry. 1. A method for producing an exhaust gas purifying catalyst having an integrated structure, characterized in that the carrier is additionally coated.

(3) The method described in (1) or (2) above, wherein the amount of rhodium supported in the final supporting step of the manufacturing process is 50% or more of the total amount of rhodium supported.

(4) The amount of supported cerium hydroxide powder is CeO, 5
~100II/l-catalyst, the method described in (1) or (2) above.

(5) Cerium hydroxide powder is a primary particle with substantially 0.1
~0.8 micron particle size and substantially 0.3 secondary particles
The method according to (1), (2) or (4) above, characterized in that the particle size is in the range of -20 microns and has a water content of 40% by weight or less.

(6) La103 of lanthanum compound together with cerium hydroxide to its total oxide (CeO2+ La201)
The method according to (1), (2), (4) or (5) above, wherein the method is used in an amount of 30% by weight or less.

(7) The zirconium compound used to disperse and support rhodium on activated alumina powder is
(1) and (2) above, characterized in that the supported amount as 02) is in the range of 0 to 10 g/13-catalyst body.
) or the method described in (3).

Scales using cerium hydroxide as a cerium compound are well known; for example, (1) JP-A-57-87839 lists it as being able to be used in the same way as other compounds such as cerium nitrate. There is. (2) JP-A No. 57-56041 describes a method in which a carrier is wash-coated, palladium is supported, fired, and then impregnated with a cerium salt aqueous solution or cerium hydroxide sol.

However, with regard to the obtained catalysts, there is little evidence that the catalyst using cerium hydroxide sol and the catalyst impregnated with an aqueous cerium salt solution exhibit almost the same activity. Furthermore, (3) JP-A No. 57-g9835 discloses that a water-soluble salt of cerium is coexisting in a dispersion of activated alumina granules in water, and aqueous ammonia is added to the dispersion to coat the surface of the alumina with cerium. A process is described which consists of precipitating the aqueous oxides and removing the soluble salts. However, this method requires filtration and washing and is a complicated process, and the resulting catalyst is characterized in that the platinum group element is supported on activated alumina containing a cerium oxide surface layer once formed.

In any case, these do not disclose the unused cerium hydroxide powder specified by the present invention, and their catalysts are inferior in activity to the catalyst of the present invention.

Hereinafter, specific embodiments of the present invention will be described.

The cerium hydroxide used in the present invention is in powder form, and particularly has a particle size of 0.1 to 0.8 μm for primary particles, preferably 0.1 to 0.4 μm, and a size of secondary particles. It is desired that the particle size is 0.3 to 20μ, preferably 0.3 to 10μ. The cerium hydroxide preferably has a water content of 40% by weight or less.

The reasons include that if the water content is too high, it becomes sticky and difficult to handle, and the particle size distribution is too fine. The water content of cerium hydroxide can be controlled to a desired value in a dehydration step during its production. The content of cerium oxide in the finished catalyst is Ce
O2 is 5-1001/l-catalyst, preferably 20-8'OI/l-catalyst. In addition, the blending amount of cerium hydroxide to activated alumina is 5 to 60
% by weight, preferably in the range 10-50% by weight.

The cerium hydroxide used preferably has a purity of 80% or more, and may contain impurities such as lanthanum oxide, neodymium oxide, praseodymium oxide, samarium oxide, etc. Other rare earth elements such as lanthanum oxide can be further added to this cerium hydroxide, but the amount added should be less than 30% by weight as La20B relative to the total amount of Ce01 and Lagol. ,
If more lanthanum oxide is contained, the observed ternary properties of the catalyst will deteriorate.

The activated alumina used in the present invention has a specific surface area of 5
Activated alumina of 0-180 trl/I is preferred.

Aluminum hydroxide, boehmite, and water-use alumina in the form of solidified boehmite can also be used if they can be supported on a honeycomb carrier and fired to become the activated alumina.

However, it must be compatible with alumina in the crystalline form of 1, δ, θ, χ, or nitric acid in the finished catalyst. Among these, particularly preferable alumina has a specific surface area of 70
˜1 eord/I activated alumina in r and δ crystal forms. Alumina is supported in an amount between 50 and 200.9 g of finished catalyst.

The catalyst is produced by the method of the present invention, for example, according to the following steps. Activated alumina is impregnated with a water-soluble compound of at least one metal of the platinum group elements, such as platinum (Pt) and palladium (PD), and these metallized metals are directly supported on the surface layer of the alumina, and then, for example, dried and calcined. Alternatively, after reducing and immobilizing the platinum group elements with a reducing agent such as hydrazine in a wet process, a predetermined amount of cerium hydroxide powder and, if necessary, a lanthanum compound are added to this, and a wet pulverizer is used. A coating liquid is prepared, supported on an integral structure carrier, excess coating liquid is blown off, for example, by air blowing, a predetermined amount of the catalyst component is supported, and then dried and calcined. Furthermore, activated alumina is impregnated with a water-soluble compound of rhodium (Rh), and the rhodium compound is directly supported on the surface layer of the alumina. If necessary, a zirconium compound may be used together with a rhodium compound, and then, for example, after drying and firing, or after reducing and fixing rhodium with a reducing agent such as hydrazine in a wet process,
A coating liquid is prepared using a wet pulverizer and supported on the coated monolithic carrier, the excess coating liquid is blown off, the desired catalyst components are supported, and the finished catalyst is dried and calcined.

Therefore, the feature of the production method of the present invention is that the platinum group elements are directly supported on activated alumina, the platinum group elements are effectively dispersed and supported on the alumina having a high surface area, and then mixed with cerium hydroxide and finely pulverized. When coated on a monolithic carrier, the cerium oxide dispersed in large amounts near the platinum group element has a stable oxygen storage ability, so when the A/P is on the rich side, the cerium oxide releases oxygen. N
It increases not only the Ox purification ability but also the CO1HC purification ability, and conversely, when the A/F is on the lean side, cerium oxide in the atmosphere increases the 00. Including Co, HC purification ability as well as NO
It provides a catalyst that improves x purification ability.

In the present invention, when reversing this oxidation-reduction state, if cerium hydroxide powder is used as a cerium source, the dispersity can be made slightly coarser than when supported with cerium nitrate or cerium hydroxide sol. Moreover, since it is dispersed in the form of cerium oxide with a predetermined particle size, there is almost no particle growth even when cerium oxide is exposed to high temperatures, and it is thought that it stably maintains and exhibits its performance.

In addition, in order to make rhodium work effectively after being supported on a monolithic carrier, especially when the amount of rhodium used is small,
A method in which cerium hydroxide is once supported on a monolithic carrier together with a platinum group element fixed on activated alumina, and then a slurry in which rhodium is effectively dispersed and supported on the activated alumina is supported on the surface layer of the monolithic carrier. is desirable. For example, after supporting and fixing PT and/or Pd on activated alumina, it is finely pulverized with cerium hydroxide to form a slurry, and this is coated on a monolithic structure carrier and fired to further activate the active alumina. The catalyst composition is made by additionally coating alumina with an aqueous powder slurry in which Rh and, if necessary, zirconium are supported and fixed, and then calcined.Because Rh is distributed near the surface, a small amount of Rh supported is particularly effective. This will help in exhaust gas purification activity.

In this case, in the method of supporting Rh near the surface layer, it has been found that when the catalyst is particularly required to have heat resistance, it is more effective to add zirconium, which is known as a heat resistance aid.

The platinum group metal used in the present invention is essentially rhodium, and platinum and /l or palladium are used, but the amount of each platinum group metal used is 1! The ratio of Rh to pt and/or Pd is 1:10.
It can be used between 0 and 1=1.

In the manufacturing method of the present invention, drying is carried out at 200°C or lower, and baking is carried out between 200 and 900°C, preferably between 400 and 800°C.

The honeycomb carrier having an integral structure used in the present invention may be one commonly called a ceramic honeycomb carrier, and in particular, cordierite, mullite, alpha alumina, zirconia, titania, titanium phosphate, aluminum titanate, petalite, Honeycomb carriers made of materials such as spodumene, aluminosilicate, and magnesium silicate are preferred, and among them, cordierite carriers are particularly preferred for use in internal combustion engines. In addition, a unitary structure made of oxidation-resistant heat-resistant metal such as stainless steel or FeClalloy is also used. These monolith carriers are manufactured by extrusion molding or by rolling and compacting sheet-like elements, and their gas passage ports (cell-shaped) also have 6
Square, four-sided, triangular, and corrugated-ylon types can be adopted. Nanada density (number of cells/unit cross-sectional area) is 150
~600 cells/square inch is fully usable and gives good results.

The present invention will be explained in more detail with reference to Examples below, but it goes without saying that the present invention is not limited only to these Examples.

Example 1 A catalyst was prepared using a commercially available cordierite monolith carrier (manufactured by NGK). The monolithic carrier has approximately 300 gas flow cells per square inch of cross-section and has an outer diameter of 33
tnl, cut into a cylindrical shape with a length of 76 fiL, approximately 65
It has a volume of -.

Dinitro-diaminoplatinum (Pi(NHs)t(Not)
z ) nitric acid aqueous solution (Pt content 1o ol/l)
Water was added to soml to make it 500-. Activated alumina powder 690.1i+ with a surface area of 1201i/i and an average particle size of 50μ was added to this aqueous solution with stirring, and after thorough mixing, it was dried at 150″C for 5 hours, and then heated to 600°C in 2 hours in an electric furnace. Firing was continued at this temperature for 2 hours, after which it was gradually cooled and taken out from the furnace.

Primary particles are in the range of 0.2 to 0.3μ, secondary particles are in the range of 1 to 6
Hydrous cerium hydroxide (Ce(OH)) in the μ range 4.4.
3H,0) powder 664I and the above baked product were mixed and milled together with dilute nitric acid water in a Pall mill for 30 hours to prepare a coating slurry.

The monolithic carrier was immersed in this coating slurry, the slurry was then taken out, and excess slurry in the cells was blown away with compressed air to remove clogging from all cells. This catalyst was dried in a dryer at 140°C for 5 hours, and then calcined in an electric furnace at 600°C for 2 hours.

On the other hand, rhodium nitrate aqueous solution [Rh (NO3) ・n
Hz O) Add activated alumina powder 4.00% as above to the liquid.
Add g while stirring, mix well, and heat to 150℃
It was dried for 5 hours, then fired in an electric furnace at 600°C for 2 hours, gradually cooled, and taken out from the furnace.

This fired product was wet-milled with dilute nitric acid water in a Pall mill to prepare a coating slurry.

The wash-coated catalyst was immersed in this coating slurry, then taken out from the slurry and blown with compressed air. This catalyst was dried in a dryer at 140° C. for 5 hours to obtain a finished catalyst. In addition, when the coating content of this catalyst was measured, it was found to be an X5OI//- catalyst. The weight ratio to the supported amount is approximately AJ t Os/C
eOx -109/4G.

For platinum and rhodium, Pt = 0.8 Ji'
/ l-catalyst and Rh = 0.08 g/l-catalyst were supported respectively [hereinafter, the ratio of supported amount is referred to as AJ20s
/CaO2/Pt/Rh-109/4010
．． Expressed as 810.08 (1/l-catalyst). ].

Comparative Example 1 A finished catalyst was prepared in substantially the same manner as in Example 1, except that cerium nitrate was used instead of cerium hydroxide as a ceria source.

That is, cerium nitrate (Ce(NOx)s -6H,O)
1010J was added with 300 gg of water to make an aqueous solution. Furthermore, a nitric acid aqueous solution of dinitro-diaminoplatinum (Pt content t
oo, @/A? ) and mixed. The same activated alumina powder 690 used in Example 1 was added to this water and solution.
After adding g and thoroughly mixing, the mixture was dried at 150°C and further calcined at 600°C for 2 hours.

This fired product was ground with dilute nitric acid water in a Pall mill to obtain a slurry for coating. Hereinafter, a finished catalyst was prepared in exactly the same manner as in Example 1. Supported amount ratio AA!
tos/CeO2/Pt/Rh=109
/4 olo, slo, os (g/g-catalyst) Example 2 Catalysts with different compositions were prepared in substantially the same manner as in Example 1. The difference from Example 1 is that instead of dinitrodiaminoplatinum, chloroplatinic acid (H2pt C
The aqueous solution of 4) was used to support 690I on activated alumina. Furthermore, before ball milling, the same hydrous cerium hydroxide [: Ce(OH)4-4.3 as used in Example 1]
At the same time, 3 g of hydrous lanthanum hydroxide (La(OH)s 2.2 HzO) powder was mixed with the powder 5301 of a and o) to prepare a coating raw material. Thereafter, wash coating was performed using the same method as in Example 1. Furthermore, as a rhodium source in the second-stage wash coat raw material, an aqueous solution of rhodium chloride (RhC1s) was used instead of an aqueous solution of rhodium nitrate, and a predetermined amount was supported in the same manner as in Example 1, and the finished catalyst was dried. did. Supported amount ratio klxos/C
eO2/La203/Pt/Rh=1
09/32/810, slo, o s (g/
l-Catalyst) Example 3 Catalysts having different noble metal compositions and heavy metal compositions were prepared in the same manner as in Example 1. The difference from Example 1 is that first, a catalyst was prepared in which palladium was supported on a platinum slag as a noble metal source. That is, dinitro-diaminoplatinum was wash-coated using a nitric acid aqueous solution of palladium nitrate (Pd(NOs)z) in the same manner as in Example 1. In addition, in the process of supporting rhodium on activated alumina, zirconyl nitrate was applied. A finished catalyst was prepared in the same manner as in Example 1 except that (Zr0(NOx)z) was added.Supported amount ratio M, o2/CeO2/ZrO,/Pd/Rh
-104/401510.810.0s (1/l-catalyst) Example 4 A catalyst different only in noble metal composition was prepared in the same manner as in Example 1. That is, a finished catalyst was prepared in the same manner as in Example 1 using a mixed solution of dinitro-diamino platinum and palladium nitrate in a dinitro-diamino platinum solution. Supported amount ratio individual g0s/CeO, /Pt/Pd/Rh -1
09/4 olo, s 710, 2310. Os (g
/l-catalyst) Example 5 The catalysts of Examples 1 to 4 and the catalyst of Comparative Example 1 were subjected to three-way characteristic evaluation experiments and light
An off-characteristic evaluation experiment was conducted.

The engine used in this experiment was a commercially available electronically controlled 4-cylinder TSOOCC. The ternary characteristic evaluation experiment is a method of modeling the A/F state of an actual closed-loop engine. Forcibly vibrate at 1.0Hz in ±o, s A/F units, and the center at that time is from person/F = 15.1 to 1
The purification rate of CO1HC and NOx components at that time was measured by changing it to 4.1 in 6 minutes, and the purification rate of the three components was plotted on the vertical axis, and the center point of the A/F was plotted on the horizontal axis. Performance was compared based on the original catalyst characteristic diagram. That is, CO
The height of the intersection of the conversion rate curve and the NOx purification rate curve [called the crossover point (COP)] is high and 80%
A/F width (
A catalyst with a large (80% window width) is a good catalyst. (The evaluation results are shown in Table 1.) On the other hand, in the light-off characteristic evaluation experiment, 1.0
The A/F was oscillated between 14.10 and 15.10 with 14.60 as the center, and the exhaust gas temperature was raised from room temperature to 450°C at a rate of 0°C/min.
The purification rates of HC and NOx components were measured, and the catalyst inlet temperature was plotted on the horizontal axis, and the purification rates of the three components were plotted on the vertical axis, and the low-temperature activity performance of the three-way catalyst was compared from the resulting diagram. In other words, the lower the temperature at which the three good catalyst components reach 50% conversion and 80% conversion (T, ., T, ., respectively), the better the catalyst has excellent low-temperature activity. be done.

Example 6 The catalyst whose initial activity was evaluated in Example 5 was subjected to endurance running using an engine bench test device. The engine used was a commercially available electronically controlled 8-cylinder 4400eC engine, and a mode operation method using a program setter was adopted as a durable operation method. That is, engine speed 2800 rp for 60 seconds
Drive steadily at mX boost pressure (-220mtxHg), then return the throttle for 6 seconds and turn off the fuel. After 6 seconds, the engine speed becomes approximately 180 rpm. During this deceleration running, the inlet oxygen concentration reaches approximately 18 degrees, causing rapid deterioration of the catalyst. A durability test was conducted by filling the catalyst into a multi-converter. Catalyst outlet temperature 750 during steady running
°C, space velocity (S, V, ) is approximately 29 J), 000 hr
The sample was aged for 100 hours under conditions such that -1 (8TP') and Person/F was 14.60.

For evaluation, ternary characteristics and light-off characteristics were measured using the same operations as in Example 5.

The evaluation results are shown in Table 1. The catalyst of the present invention possesses the highest level of COP and effective window in ternary properties. Furthermore, in terms of light-off characteristics, the catalyst of the present invention is superior to Comparative Example 1 since TSo s T2O is at a low temperature.

Claims (7)

[Claims] (1) At least one platinum group element compound is dispersed and supported on activated alumina powder, the platinum group element is fixed on the activated alumina, and this is further formed into an aqueous slurry with cerium hydroxide powder, resulting in a slurry composition. After that, a rhodium compound is dispersed and supported on activated alumina powder, rhodium is fixed on the activated alumina, and then an aqueous slurry is formed, and the resulting slurry is coated on the coated support. A method for producing an integrated structure exhaust gas purifying catalyst characterized by additionally coating the catalyst. (2) At least one platinum group element compound is dispersed and supported on activated alumina powder, the platinum group element is fixed on the activated alumina, and this is further formed into an aqueous slurry with cerium hydroxide powder, resulting in a slurry composition. After that, a rhodium compound and a zirconium compound are dispersed and supported on activated alumina powder, rhodium and zirconia are fixed on the activated alumina, and this is made into an aqueous slurry. A method for producing an integrated structure exhaust gas purifying catalyst, which comprises additionally coating the coated carrier. (3) The method according to claim (1) or (2), wherein the amount of rhodium supported in the final supporting step of the manufacturing process is 50% or more of the total amount of rhodium supported. (4) The method according to claim (1) or (2), characterized in that the amount of cerium hydroxide powder supported is 5 to 100 g/l of CeO_2 catalyst. (5) Cerium hydroxide powder is a primary particle with substantially 0.1
~0.8 micron particle size and substantially 0.3 secondary particles
The method according to claim (1), (2) or (4), characterized in that the particle size is in the range of ~20 microns and has a water content of 40% by weight or less. . (6) La_2 for the total oxide (CeO_2+La_2O_3) of lanthanum compound together with cerium hydroxide
Claims (1), (2), (4) or (
5) The method described. (7) The zirconium compound used when rhodium is dispersed and supported on activated alumina powder is its oxide (ZrO_
Claim (1) characterized in that the supported amount as 2) is made to be in the range of 0 to 10 g/l-catalyst body,
The method described in (2) or (3).