JPH052373B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a monolithic catalyst for exhaust gas purification. To be more specific, the present invention deals with harmful components contained in exhaust gas, such as hydrocarbons (hereinafter referred to as HC) and carbon monoxide (hereinafter referred to as CO).
The present invention also relates to a method for producing a monolithic catalyst for removing nitrogen oxides (hereinafter referred to as NOx). More specifically, the present invention provides a method for reducing the amount of HC, CO, and NOx in the exhaust gases of an internal combustion engine when it is operated near the stoichiometric point of air-to-fuel ratio.
At the same time, it can be made stable and substantially harmless, and
The present invention relates to a method for producing an integrated structure catalyst for exhaust gas purification that exhibits little deterioration even when exposed to high temperatures of 800°C or higher. HC, CO and NOx3 in exhaust gas from internal combustion engines
Catalysts that simultaneously remove components using a single 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 increased, partly as a measure to improve fuel efficiency. are doing. In this case, the catalyst is often installed under the floor, and in some cases it is installed directly under the engine manifold. When an engine equipped with this three-way catalytic converter is operated near the stoichiometric air-fuel ratio (A/F), it will emit exhaust gas that has purified the three components mentioned above most effectively. In order to make it work more effectively, a system has been adopted in which the A/F is controlled using an electronically controlled fuel injection device or a ventilate carburetor that supplies fuel using an injection pump to maintain a constant A/F. However, depending on the control method, A/
F may vary over a fairly wide range away from the stoichiometric equivalence point, and in the case of sudden changes in operation such as acceleration or deceleration, this is to prevent melting due to a sudden rise in temperature of the monolithic catalyst. In some cases, the fuel supply may be partially or completely cut off and the vehicle 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, there is a need for a three-way catalyst that exhibits stable purification performance even when operated at a wide range of A/F and exhibits less deterioration. 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 as a substance with oxygen storage ability in coexistence with a platinum metal element. A method of supporting activated alumina in the form of salts 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 acetate has the drawback of poor durability, especially poor heat resistance. Further, JP-A-58-122044 describes a method in which the raw material for the catalyst component is used in the form of a carbonate, such as 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. This has the drawback that the slurry has strong thixotropic properties, making it difficult to coat smoothly as a coating composition. 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) 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 substance is coated on a monolithic structure carrier, and then a rhodium compound is dispersed and supported on activated alumina powder to fix rhodium on the activated alumina, and then an aqueous slurry is formed, and the resulting slurry is coated with the above-mentioned coating. 1. A method for producing an integrated structure exhaust gas purifying catalyst, which comprises additionally coating the catalyst. (2) The method according to claim 1, 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. (3) The method according to claim 1, wherein the amount of cerium hydroxide powder supported is 5 to 100 g/-catalyst as _CeO2 . (4) Cerium hydroxide powder is essentially a primary particle.
Claim 1 or 3, which has a particle size of 0.1 to 0.8 microns and a secondary particle size of substantially 0.3 to 20 microns, and has a water content of 40% by weight or less. Method described. (5) Claims 1, 3 or 4 in which a lanthanum compound is used together with cerium hydroxide in an amount of 30% by weight or less as La ₂ O ₃ based on the total oxide (CeO ₂ + La ₂ O ₃ ). Method described. (6) 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 obtained by coating a substance on a monolithic carrier, then dispersing and supporting a rhodium compound and a zirconium compound on activated alumina powder, fixing rhodium and zirconia on the activated alumina, and then turning this into an aqueous slurry. A method for producing an integrated structure exhaust gas purifying catalyst, comprising additionally coating the coated carrier. (7) The method according to claim 6, 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. (8) The method according to claim 6, wherein the amount of cerium hydroxide powder supported is 5 to 100 g/-catalyst as _CeO2 . (9) Cerium hydroxide powder is essentially a primary particle.
Claim 6 or 8, which has a particle size of 0.1 to 0.8 microns and a secondary particle having a particle size of substantially 0.3 to 20 microns, and has a water content of 40% by weight or less. Method described. (10) Claims 6, 8 or 9 in which a lanthanum compound is used together with cerium hydroxide in an amount of 30% by weight or less as La ₂ O ₃ based on the total oxide (CeO ₂ + La ₂ O ₃ ). Method described. (11) Claim 6, wherein the zirconium compound used when dispersing and supporting rhodium on activated alumina powder is capable of supporting an amount of 0 to 10 g/-catalyst as its oxide (ZrO ₂ ). or the method described in 7. It is known that cerium hydroxide can be used as a cerium compound; 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 an aqueous cerium salt solution or cerium hydroxide sol. However, it is merely stated that the obtained catalysts exhibit almost the same activity as those using cerium hydroxide sol and those impregnated with an aqueous cerium salt solution. Furthermore, (3) JP-A-57-119835 discloses that a water-soluble salt of cerium is coexisting in a dispersion of activated alumina granules dispersed in water, and aqueous ammonia is added to the dispersion to coat the surface of the alumina with cerium. A method is described which consists of precipitating the hydrated oxide and removing the soluble salts. However, this method requires filtering and washing, and the process is complicated, and the resulting catalyst is characterized in that the platinum group element is supported on the activated alumina containing the cerium oxide surface layer once formed. In any case, they do not disclose the use of cerium hydroxide powder as 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 explained. The cerium hydroxide used in the present invention is in powder form, and the particle size is particularly large as the primary particle size is 0.1 to 1.
It is desired that the particle size is 0.8μ, preferably 0.1 to 0.4μ, and the size of the secondary particles is 0.3 to 20μ, preferably 0.3 to 10μ. Further, 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 becomes 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 5 to 100 g/-catalyst body as _CeO2 , preferably 20 to 80 g.
g/- catalyst body. In addition, the blending amount of cerium hydroxide to activated alumina is 5 to 60% by weight,
Preferably it is in the range of 10 to 50% by weight. The cerium hydroxide used preferably has a purity of 80% or more, and impurities include lanthanum oxide,
Neodymium oxide, proseodymium oxide, samarium oxide, etc. may be contained. Other rare earth elements, such as lanthanum oxide, can be further added to this cerium hydroxide, but the amount added is the same as La ₂ O ₃ with respect to the total amount of CeO ₂ and La ₂ O ₃ .
It should be less than 30% by weight; if more lanthanum oxide is contained, the resulting catalyst will have poor ternary properties. The activated alumina used in the present invention preferably has a specific surface area of 50 to 180 m ² /g.
Aluminum hydroxide, boehmite, and hydrated alumina in the form of solidified boehmite can also be used if they can be formed into the activated alumina by supporting a honeycomb carrier and then firing. However, in the finished catalyst, γ,
It must be capable of forming alumina in the δ, θ, χ, κ, or η crystal form. Of these,
Particularly preferable alumina has a specific surface area of 70 to 160
m ² /g of activated alumina in γ and δ crystalline forms. Alumina is supported in an amount between 50 and 200 grams per 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 metal compounds are directly supported on the surface layer of the alumina, and then, for example, dried and fired. mosquito,
After reducing and fixing 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 coating liquid is prepared using a wet pulverizer. is prepared, supported on a monolithic carrier, excess coating liquid is blown away, for example, by air blowing, a predetermined amount of catalyst component is supported, 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, the rhodium may be dried and fired, or the rhodium may be reduced and fixed with a reducing agent such as hydrazine in a wet process, and then the coating liquid may be prepared using a wet pulverizer. The prepared catalyst is supported on the coated monolithic carrier, the excess coating liquid is blown off, the desired catalyst components are supported, and the catalyst is dried and fired to obtain a finished catalyst. 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 the cerium oxide is dispersed in large quantities near the platinum group element and coated on an integral structure carrier, it has a stable oxygen storage ability, so when the A/F is on the rich side, oxygen can be absorbed from the cerium oxide. release,
Improves not only NOx purification ability but also CO and HC purification ability,
Conversely, when the A/F is on the lean side, cerium oxide takes in O ₂ from the atmosphere and provides a catalyst that improves not only CO and HC purification ability but also NOx purification ability. In the present invention, when this oxidation-reduction state is repeated, if cerium hydroxide powder is used as the cerium source, the degree of dispersion can be made slightly coarser than in the case of supporting with cerium nitrate or cerium hydroxide sol. Because the cerium oxide is dispersed in the form of cerium oxide with a particle size of In addition, after being supported on the monolithic structure carrier, in order to make rhodium work effectively especially when the amount of rhodium used is small, cerium hydroxide was once supported on the monolithic structure carrier together with the platinum group element fixed on activated alumina. Afterwards, it is desirable to further support a slurry in which rhodium is effectively dispersed and supported on activated alumina on the surface layer of the monolithic support. For example, after supporting and fixing Pt and/or Pd on activated alumina, it is finely pulverized with cerium hydroxide to form a slurry, coated on a monolithic structure carrier, and then applied to the surface of a fired catalyst composition, and then applied to activated alumina. The catalyst composition, which is additionally coated with an aqueous slurry of powder in which Rh and, if necessary, zirconium are supported and fixed, is calcined.
This will particularly effectively contribute to exhaust gas purification activity. In this case, when the catalyst is particularly required to have heat resistance in the method of supporting Rh near the surface layer,
It has been found that the addition of zirconium, which is known as a heat resistance additive, is also effective. The platinum group metal used in the present invention is essentially rhodium, and platinum and/or palladium are used, but the amount of each platinum group metal used is per catalyst.
Used between 0.1 and 5g, Rh and Pt and/or
Pd ratios between 1:100 and 1:1 can be used. In the manufacturing method of the present invention, drying is carried out at a temperature of 200°C or lower, and baking is carried out at a temperature of 200 to 900°C, preferably a temperature of 400 to 800°C. The honeycomb carrier having a monolithic structure used in the present invention may be one commonly referred to as a ceramic honeycomb carrier, particularly cordierite, mullite, alpha alumina, zirconia, titania, titanium phosphate, aluminum titanate,
Honeycomb carriers made of petalite, spodium, aluminosilicate, magnesium silicate, etc. are preferred, and among them, cordierite is 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 holes (cell-shaped) are also hexagonal, square, square, etc.
Either triangular or corrugation type can be adopted. Cell 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 monolith carrier has about 300 gas flow cells per square inch of cross section, and is cut into a cylindrical shape with an outer diameter of 33 mm and a length of 76 mm, and has a volume of about 65 ml. Dinitro-diaminoplatinum [Pt(NH ₃ ) ₂ (NO ₂ ) ₂ ]
Water was added to 80 ml of nitric acid aqueous solution (Pt content 100 g/) to make 500 ml. This aqueous solution has a surface area of 120 m ² /
g, 690 g of activated alumina powder with an average particle size of 50 μm was added with stirring, and after thorough mixing, it was heated at 150°C x 5
After drying for an hour, the temperature was raised to 600° C. over 2 hours in an electric furnace, and 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 0.3μ
Hydrated cerium hydroxide [Ce(OH) ₄・
4.3H ₂ O] and the above baked product were mixed and wet-milled together with dilute nitric acid water in a ball mill for 30 hours to prepare a coating slurry. The monolithic carrier was immersed in this coating slurry, then taken out from the slurry, and excess slurry in the cells was blown out with compressed air to remove clogging from all cells. This catalyst was heated to 140℃ x 5
Dry in an oven for an hour, then in an electric oven at 600℃ x 2
Baked for an hour. On the other hand, rhodium nitrate aqueous solution [Rh(NO ₃ ) ₃ .
Add water to 16 ml of nH ₂ O] (Rh content 5 g/) and add 300
ml. 400g of the same activated alumina powder as above was added to this aqueous solution with stirring, and after thorough mixing, it was dried at 150℃ for 5 hours, and then heated to 600℃ in an electric furnace.
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 ball mill to prepare a slurry for coating. The wash-coated catalyst was immersed in this coating slurry, then taken out from the slurry and blown with compressed air. This catalyst
It was dried in a dryer at 140°C for 5 hours to obtain a completed catalyst.
In addition, when the coating amount of this catalyst was measured, it was 150 g/-catalyst. The weight ratio to the supported amount was approximately Al ₂ O ₃ /CeO ₂ = 109/40. Regarding platinum and rhodium, Pt = 0.8 g/- catalyst and Rh = 0.08 g/- catalyst were supported respectively [hereinafter, this supported amount ratio will be expressed as Al ₂ O ₃ /CeO ₂ /Pt/
Expressed as Rh=109/40/0.8/0.08 (g/-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(NO ₃ ) _{3.6H 2} O]
300 ml of water was added to 1010 g to make an aqueous solution. In addition, a nitric acid aqueous solution of dinitro-diaminoplatinum (Pt content
100g/) was added and mixed. The same activated alumina powder 690 used in Example 1 was added to this aqueous 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 ball 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 Al ₂ O ₃ /CeO ₂ /Pt/Rh=109/
40/0.8/0.08 (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, an aqueous solution of chloroplatinic acid [H ₂ PtCl ₆ ] was used as the noble metal source and supported on 690 g of activated alumina. Furthermore, before ball milling, 530 g of the same hydrous cerium hydroxide [Ce (OH ₄ 4.3H ₂ O)] powder used in Example 1 was simultaneously added to 530 g of hydrated lanthanum hydroxide [La (OH) ₃ 2.2H _{2 ]} . 113g of powder of [O]] was mixed to prepare a coating material.
Thereafter, wash coating was performed using the same method as in Example 1. Furthermore, an aqueous solution of rhodium chloride [RhCl ₃ ] was used instead of an aqueous solution of rhodium nitrate as the rhodium source in the second-stage wash coat raw material.
Thereafter, a predetermined amount of the catalyst was supported in the same manner as in Example 1 and dried to obtain a finished catalyst. Supported amount ratio Al ₂ O ₃ /CeO ₂ /
La ₂ O ₃ /Pt/Rh=109/32/8/0.8/0.08 (g/
-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 was that first, a catalyst was prepared in which palladium was supported instead of platinum as a noble metal source. Immediately, instead of dinitrodiaminoplatinum, palladium nitrate [Pd
Wash-coating was performed in the same manner as in Example 1 using an aqueous nitric acid solution of (NO ₃ ) ₂ ]. Example 1 except that zirconyl nitrate [ZrO(NO ₃ ) ₂ ] was added in the step of further supporting rhodium on activated alumina.
The finished catalyst was prepared using the same method as described above. Supported amount ratio Al ₂ O ₃ /CeO ₂ /ZrO ₂ /Pd/Rh=104/40/
5/0.8/0.08 (g/-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 instead of dinitro-diamino platinum. Supported amount ratio Al ₂ O ₃ /
CeO2 _/ Pt/Pd/Rh=109/40/0.57/0.23/0.08
(g/-catalyst) Example 5 Three-way characteristic evaluation experiments and light-off characteristic evaluation experiments were conducted on the catalysts of Examples 1 to 4 and the catalyst of Comparative Example 1 using engine exhaust gas. The engine used in this experiment was a commercially available electronically controlled 4-cylinder 1800c.c. The ternary characteristic evaluation experiment is a method of modeling the A/F state of an actual closed-loop engine. Forced ± at 1.0Hz
Vibrate at a rate of 0.5A/F, and set the center at A/F.
F=15.1 to 14.1 was changed in 6 minutes and the purification rate of CO, HC and NOx components was measured.
Performance was compared based on a three-way catalyst characteristic diagram created by plotting the purification rates of the three components on the vertical axis and the center point of A/F on the horizontal axis. In other words, the intersection point of the CO conversion rate curve and the NOx purification rate curve
point (COP)] is high and 80
A good catalyst is one with a large A/F width (80% window width) that can simultaneously achieve a conversion rate of % or more and a purification rate. (The evaluation results are shown in Table 1.) On the other hand, in the light-off characteristic evaluation experiment, the A/F was forcibly adjusted from 14.10 to 14.60 at 1.0Hz.
15.10 to raise the exhaust gas temperature from room temperature to 450°C at a rate of approximately 10°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 graph. In other words, the temperature at which all three good catalyst components reach 50% conversion and 80% conversion (respectively
The lower the T ₅₀ and T ₈₀ (shown in Table 1), the better the catalyst has low-temperature activity. 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 is a commercially available electronically controlled 8-cylinder 4400c.c.
As a durable operation method, a mode operation method using a program setter was adopted. That is, engine speed 2800 rpm x boost pressure (-220, mm) for 60 seconds
Ride at a steady state (Hg), then return the throttle for 6 seconds and turn off the fuel. After 6 seconds, the engine speed will be approximately
It becomes 1800rpm. During this deceleration, the inlet oxygen concentration reaches approximately 18%, causing rapid deterioration of the catalyst. The catalyst was packed into a multi-converter and a durability test was conducted. Catalyst outlet temperature 750℃ during steady running.
Space velocity (SV) is approximately 290000hr ^-1 (STP), A/F
It was aged for 100 hours under the condition that 14.60. The evaluation was carried out in the same manner as in Example 5.
The ternary characteristics and light-off characteristics were measured. 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 has lower T ₅₀ and T ₈₀ and is superior to Comparative Example 1. 【table】

Claims (1)

[Claims] 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 this is further formed into an aqueous slurry with cerium hydroxide powder. The resulting slurry composition is coated on an integral structure carrier, and then a rhodium compound is dispersed and supported on activated alumina powder to fix rhodium on the activated alumina, and then an aqueous slurry is formed, and the resulting slurry is coated with the coating. A method for producing an integrated structure exhaust gas purifying catalyst, which comprises additionally coating the treated carrier. 2. The method according to claim 1, 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. 3 Claim 1 in which the amount of cerium hydroxide powder supported is 5 to 100 g/-catalyst as CeO ₂
Method described. 4 Cerium hydroxide powder is a primary particle and is substantially
Particle size from 0.1 to 0.8 microns and substantially secondary particles
4. A method according to claim 1, wherein the particle size ranges from 0.3 to 20 microns and the water content is less than 40% by weight. 5 Calculate lanthanum compounds together with cerium hydroxide as La ₂ O ₃ for the total oxide (CeO ₂ + La ₂ O ₃ )
5. The method according to claim 1, 3 or 4, wherein the amount is 30% by weight or less. 6 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, and the resulting slurry composition is A monolithic structure carrier is coated, 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 then 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. 7. The method according to claim 6, 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. 8 Claim 6 wherein the amount of cerium hydroxide powder supported is 5 to 100 g/-catalyst as CeO ₂
Method described. 9 Cerium hydroxide powder is a primary particle and is substantially
Particle size from 0.1 to 0.8 microns and substantially secondary particles
9. A method according to claim 6, wherein the particle size ranges from 0.3 to 20 microns and the water content is less than 40% by weight. 10 Claim 6, 8 or 9, wherein a lanthanum compound is used together with cerium hydroxide in an amount of 30% by weight or less as La ₂ O ₃ based on the total oxide (CeO ₂ + La ₂ O ₃ ). Method. 11 Claim 6 or Claim 6, wherein the zirconium compound used to disperse and support rhodium on activated alumina powder has a supported amount in the form of an oxide (ZrO ₂ ) of 0 to 10 g/-catalyst. 7. The method described in 7.