JPH0573465B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to an exhaust gas purifying catalyst. Specifically, the present invention provides an exhaust gas that simultaneously removes hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx), which are harmful components contained in exhaust gas from internal combustion engines such as automobiles. It relates to purification catalysts, and has excellent durability even when used under harsh conditions such as high-temperature oxidizing atmospheres, and also has high purification performance at low temperatures against the above-mentioned harmful components. It is related to. [Prior art] Conventionally, in exhaust gas purification catalysts, efforts have been made to support precious metals such as activated alumina with a high surface area as highly as possible in order to effectively use precious metals whose usage is limited to a very small amount. It has been done. However, although catalysts with highly dispersed noble metals supported have high initial activity, when exposed to harsh conditions such as high-temperature oxidizing atmospheres, noble metal particles are likely to grow and reactions between noble metals and support substances occur. Because it is supported in a highly dispersed manner, there was a problem in that the deterioration of activity was rather large. In this field, zirconia is often used for the purpose of stabilizing physical properties such as the specific surface area of catalysts, but examples of its use as a support material for precious metals include Japanese Patent Publication No. 57-29215 and Japanese Patent Application Laid-Open No. 57−
No. 153737 proposes a method in which a coating layer containing alumina and zirconia is formed on a support and then a noble metal is supported. However, in these methods, since most of the noble metals are substantially highly dispersed in alumina, activity deterioration occurs due to the same causes as described above. U.S. Pat. No. 4,233,189 discloses a catalyst in which rhodium is supported at a low loading rate on zirconia after forming a coating layer of zirconia on a carrier, but the noble metal disclosed in the present invention is supported at a high loading rate on zirconia. The durability of the catalyst was significantly inferior to that of a catalyst in which a small amount of supported zirconia was dispersed in a large amount of other refractory inorganic oxide. [Means for Solving the Problems] As a result of intensive research, the present inventors have found that noble metals, whose usage is limited to a trace amount, are supported at a low loading rate on a large amount of refractory inorganic oxide with a high surface area. Quite contrary to the conventional knowledge that the degree of dispersion of precious metals should be increased as much as possible, zirconia containing precious metals, in which precious metals are supported at a high loading rate on a small amount of zirconia whose physical properties have been specified, has an average particle size of 0.5 to 20μ. They discovered that the durability of the catalyst could be dramatically improved by adjusting the agglomerated particles to a relatively large size and dispersing them in the catalyst coating layer, leading to the completion of the present invention. Furthermore, the present inventors have discovered that when platinum and/or palladium and rhodium are co-existed on a refractory inorganic oxide at the high loading rate disclosed in the present invention, each of the platinum and/or palladium and rhodium is It has also been found that the chemical change of precious metals into an inactive state is suppressed. This also applies to exhaust gas purification catalysts.
This was completely unforeseen from the conventional knowledge that when platinum, palladium, and rhodium are closely present, they form an alloy and lose their activity. [Structure of the Invention] The present invention is specified as follows. (1) Contains zirconia (a) on which platinum and/or palladium is supported in a range of 5 to 30% by weight and rhodium in a range of 1 to 20% by weight in the form of agglomerated particles with an average particle size of 0.5 to 20μ. A catalyst composition further containing cerium oxide (b) and optionally a refractory inorganic oxide (c) is coated and supported on a honeycomb carrier having an integral structure, and the zirconia (a) is at least 10m ² /
1. A catalyst for exhaust gas purification, characterized in that it has a specific surface area of 2,000 Å or less, and the average particle diameter of its primary particles is 2,000 Å or less. (2) 1 zirconia (a) per 1 carrier
~20g of the cerium oxide (b), 1~150g of the cerium oxide (b) as _CeO2 , and further 50~200g of the refractory inorganic oxide (c) as required. Catalyst according to scope 1. (3) The cerium oxide (b) is alumina-modified cerium obtained by impregnating a water-insoluble cerium compound with at least one selected from the group consisting of a water-soluble aluminum compound and alumina hydrate and firing the impregnated water-insoluble cerium compound. The catalyst according to claim 1 or 2, which is an oxide. (4) The water-insoluble cerium compound is selected from the group consisting of cerium carbonate, cerium oxide, and cerium hydroxide, and the water-soluble aluminum compound is selected from the group consisting of aluminum nitrate, aluminum chloride, and aluminum sulfate. 4. Catalyst according to claim 3, characterized in that it is selected from the group consisting of: (5) The composition of the alumina-modified cerium oxide is the atomic ratio of cerium to aluminum.
The catalyst according to claim 3 or 4, characterized in that the Ce/Al ratio is 1 to 20. The configuration of the present invention will be explained in detail below. Firstly, the range of high loading of precious metals on zirconia is 5% for platinum and/or palladium.
~30% by weight, preferably 10-20% by weight, and for rhodium 1-20% by weight, preferably 1-10% by weight. If the content of platinum and/or palladium is less than 5% by weight, or if the content of rhodium is less than 1% by weight, the content will approach a normal highly dispersed state, resulting in significant deterioration of activity. In addition, if platinum and/or palladium exceeds 30% by weight or rhodium exceeds 20% by weight, the number of active sites of noble metals that effectively contribute to the reaction will not increase, but will decrease from the initial stage, resulting in The performance is low, and noble metal particle growth occurs, which is not observed when the loading ratio is within the range of the present invention, resulting in the particles becoming huge and the activity of the catalyst being significantly reduced. Further, by supporting both platinum and/or palladium and rhodium at a high loading rate, durability performance is further improved. This is thought to be because the interactions between the respective noble metals suppress irreversible chemical changes to an inactive state, such as when rhodium forms rhodium oxide, which is difficult to reduce. Surprisingly, within the loading ratio range of the present invention, no deactivation due to alloying of platinum, palladium, and rhodium was observed. Next, it is essential to disperse zirconia with a high loading rate of precious metals into a large amount of other refractory inorganic oxides in the form of agglomerated particles with a relatively large average particle size of 0.5 to 20μ. This is a feature of the invention. By setting the average particle size in this range,
The interaction and reaction between the noble metal and the refractory inorganic oxide can be alleviated without impairing the efficiency of the exhaust gas purification reaction. The zirconia used in the present invention is at least
Those having a specific surface area of 10 m ² /g, preferably 60 to 100 m ² /g, and an average primary particle size of 2000 Å or less, preferably 500 Å or less are suitable. Zirconia having the above physical properties may be commercially available, or may be prepared by neutralizing an aqueous solution of zirconium salt with ammonia or the like, washing with water, drying, and firing. . Additionally, zirconia stabilized with up to 10% by weight of an alkaline earth metal such as yttrium or calcium can also be used. Such zirconia provides a good supporting state when noble metal is supported at a high loading rate. Supporting of noble metals is carried out by mixing a noble metal solution with zirconia, drying, firing, and reducing if necessary. In this operation, a mixed solution of multiple noble metal solutions may be used, or a mixture of multiple noble metal solutions may be used. The operation of mixing each type with zirconia and drying may be repeated. At this time, since zirconia has a large particle size as agglomerated particles, it is pulverized with a mill or the like to have an average particle size of 0.5 to 20 μm. The thus obtained slurry containing the noble metal-containing zirconia whose particle size has been adjusted is wash-coated onto a honeycomb carrier having an integral structure, dried and, if necessary, calcined to obtain a finished catalyst. The starting material for the cerium oxide source used in the present invention is not particularly limited as long as it can exist as cerium dioxide (CeO ₂ ) in the catalyst. For example, commercially available CeO ₂ , cerium carbonate, cerium hydroxide, etc. can be used. Alternatively, a solution of a cerium salt, such as a cerium nitrate solution, may be impregnated and supported with a refractory inorganic oxide. However, by using as ceria an alumina-modified cerium oxide obtained by impregnating a water-insoluble cerium compound with at least one selected from the group consisting of a water-soluble aluminum compound and alumina hydrate and firing the impregnated water-insoluble cerium compound. , the catalyst according to the invention shows better performance. Examples of the water-insoluble cerium compound include cerium oxide, cerium hydroxide, and cerium carbonate, with cerium carbonate being particularly preferred. This water-insoluble cerium compound is used in the form of fine powder, and its particle size is 0.1 to 100μ. In addition, water-soluble aluminum compounds and/or alumina hydrates include:
Examples include aluminum nitrate, aluminum chloride, aluminum sulfate, gypsite, bayerite, boehmite, alumina gel, alumina sol, etc.
Particularly preferred is aluminum nitrate. The ratio of the water-insoluble cerium compound to the water-soluble aluminum compound and/or alumina hydrate is not particularly limited, and an effectively alumina-modified cerium oxide can be obtained, but the atomic ratio of cerium and aluminum is preferably adjusted. So, Ce/Al=
Ce/Al=1 to 20, particularly preferably Ce/Al=2 to 10. After the water-insoluble cerium compound is impregnated with the water-soluble aluminum compound and/or alumina hydrate, it is usually dried at 100 to 300℃, and then dried at 300 to 700℃ in air.
An alumina-modified cerium oxide is obtained by firing. The thus obtained slurry containing noble metal-containing zirconia and cerium oxide whose particle size has been adjusted is wash-coated onto a honeycomb carrier having an integral structure, dried and, if necessary, calcined to obtain a finished catalyst. A honeycomb carrier having an integral structure refers to a ceramic carrier such as cordierite, mullite, or α-alumina, or a metal monolith carrier using an oxidation-resistant heat-resistant metal such as stainless steel or Fe-Cr or Al alloy. As the noble metal source used in the present invention, chloroplatinic acid, dinitrodiammine platinum, palladium chloride, palladium nitrate, rhodium chloride, rhodium nitrate, rhodium sulfate, etc. are preferable. The refractory inorganic oxides used in the present invention include alumina, silica, titania, zirconia,
Examples include magnesia, but alumina, particularly activated alumina, is preferably used. Any of γ, δ, θ, α, χ, κ, and η crystal forms of alumina can be used. In addition, alkaline earth elements such as calcium and barium, rare earth elements such as lanthanum and cerium, as well as chromium, manganese,
At least one metal element such as iron, cobalt, nickel, or zirconium in the form of an oxide
Activated alumina loaded at 0.1-30% by weight can also be used. EXAMPLES Hereinafter, the present invention will be explained in more detail with reference to Examples, 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 Nippon Insulators Co., Ltd.). The monolithic support used had a cross section of approximately
Outer diameter 33mmφ with 400 gas flow cells
It was cylindrical and had a volume of approximately 65 ml. A mixed solution of a nitric acid aqueous solution of dinitrodiamine platinum containing 1.5 g of platinum (Pt) and a rhodium nitrate aqueous solution containing ^0.3 g of rhodium (Rh) was mixed with zirconia (No. 7.5 g of Ichiki Genso Kagaku Co., Ltd.) were mixed and dried at 120°C overnight. Thereafter, it was fired in air at 400° C. for 2 hours to prepare a zirconia powder containing 16.1% by weight of Pt and 3.2% by weight of Rh. The above Pt- and Rh-containing zirconia powder was ground in a mortar so that the average particle diameter of the aggregated particles was about 20 μ, and then 139 g of activated alumina with a specific surface area of 100 m ² /g and commercially available cerium oxide powder (manufactured by Nissan Kigenshi Co., Ltd.) were ground. ) and wet milling in a ball mill for 20 hours to prepare an aqueous coating slurry. The monolithic carrier was immersed in this coating aqueous slurry, and after being taken out, the excess slurry in the cell was blown out with compressed air to remove all clogging. The catalyst was then dried at 130°C for 3 hours to obtain a finished catalyst. The coating layer of this catalyst is exposed to the Electron Probe.
Micro Analysis (EPMA) was used to take and analyze Pt and Ph distribution photographs at 3000x magnification at 30 random locations, and it was found that Pt and Ph-containing zirconia were dispersed with an average particle size of 7μ. In addition, this catalyst has Pt0.065g per piece,
It contained Ph0.013g. Example 2 A mixed solution of a palladium nitrate aqueous solution containing 1.5 g of palladium (Pd), a rhodium nitrate aqueous solution containing 0.3 g of rhodium, and 7.5 g of zirconia used in Example 1 were mixed and dried at 120°C overnight. did.
Thereafter, it was fired in air at 400° C. for 2 hours to prepare a zirconia powder containing 16.1% by weight of Pd and 3.2% by weight of Rh. A completed catalyst was obtained in the same manner as in Example 1 except that the Pd and Rh containing zirconia powder was used instead of the Pt and Rh containing zirconia powder. When the coating layer of this catalyst was analyzed by EPMA, it was found that Pd and Rh-containing zirconia were dispersed with an average particle size of 3μ. In addition, this catalyst has Pd0.065g per piece,
It contained 0.013g of Rh. Example 3 Chloroplatinic acid aqueous solution containing 0.9 g of Pt and 0.6 g of Pd
Palladium chloride aqueous solution containing Ph0.3g
A mixed solution of rhodium nitrate aqueous solution containing
and dried overnight. Thereafter, it was fired in air at 400° C. for 2 hours to prepare a zirconia powder containing 9.5% by weight Pt, 6.5% by weight Pd, and 3.2% by weight Rh. A finished catalyst was obtained in the same manner as in Example 1, except that the Pt, Pd and Rh containing zirconia powder was used instead of the Pt and Rh containing zirconia powder. When the coating layer of this catalyst was analyzed by EPMA, it was found that Pt, Pd, and Rh-containing zirconia were dispersed with an average particle size of 13μ. This catalyst has Pt0.039g, Pd0.026g per piece,
It contained 0.013g of Rh. Example 4 Completed in the same manner as Example 1 except that zirconia (manufactured by Daiichi Kigenso Kagaku) having a specific surface area of 90 m ² /g and an average particle diameter of 150 Å was used instead of the zirconia used in Example 1. I got a catalyst. When the coating layer of this catalyst was analyzed by EPMA, it was found that Pt and Rh-containing zirconia were dispersed with an average particle size of 2 μm. Each catalyst contained 0.065 g of Pt and 0.013 g of Rh. Example 5 Thin plates of ferrite stainless steel with a thickness of 60μ and containing 5% by weight of aluminum and corrugated plates formed from these thin plates into a corrugated shape with a pitch of 2.5 mm were alternately stacked and laminated to give an outer diameter of 33 mmφ and a length. A 76 mm cylindrical metal monolith carrier was molded. The carrier had a cross section of approximately 475 gas flow cells per square inch. A completed catalyst was obtained in the same manner as in Example 1 except that the above metal monolith support was used instead of the cordierite monolith support. When the coating layer of this catalyst was analyzed by EPMA, it was found that Pt and Rh-containing zirconia were dispersed with an average particle size of 6 μm. Each catalyst contained 0.065 g of Pt and 0.013 g of Rh. Example 6 25.2 g of cerium nitrate [Ce(NO ₃ ) ₃ ·6H ₂ O] and 10.1 g of iron nitrate [Fe(NO ₃ ) ₃ ·9H ₂ O] were dissolved in 100 g of pure water, and the specific surface area was 100 m ² /g. 127g of activated alumina
and dried at 120°C overnight. Thereafter, it was calcined in air at 700° C. for 1 hour to obtain alumina powder containing CeO ₂ and Fe ₂ O ₃ . A finished catalyst was obtained in the same manner as in Example 1, except that the above CeO ₂ and Fe ₂ O ₃ -containing alumina was used instead of 139 g of activated alumina. When the coating layer of this catalyst was analyzed by EPMA, it was found that Pt and Rh-containing zirconia were dispersed with an average particle size of 5 μm. In addition, this catalyst has Pt0.065g per piece,
It contained 0.013g of Rh. Example 7 Aluminum nitrate [Al(NO ₃ ) ₃・9H ₂ O] 65.3g
Thoroughly mix 150 ml of an aqueous solution with 319 g of cerium carbonate powder (Ce content: 47% by weight as CeO ₂ ), dry at 130°C for 5 hours, and then dry in air.
It was fired at 500°C for 1 hour to prepare alumina-modified cerium oxide (Ce/Al=5 atomic ratio). A completed catalyst was obtained in the same manner as in Example 1, except that 75 g of the alumina-modified cerium oxide obtained above was used instead of the commercially available cerium oxide powder. When the coating layer of this catalyst was analyzed by EPMA, it was found that Pt-containing zirconia and Rh-containing zirconia were each dispersed with an average particle size of 3 μm. Each catalyst contained 0.065 g of Pt and 0.013 g of Rh. Example 8 Aluminum nitrate [Al(NO ₃ ) ₃・9H ₂ O] 54.4 g
Thoroughly mix 220 ml of an aqueous solution with 426 g of cerium carbonate powder (Ce content: 47% by weight as CeO ₂ ), dry at 130°C for 5 hours, and dry in air.
It was fired at 500° C. for 1 hour to prepare alumina-modified cerium oxide (Ce/Al=8 atomic ratio). A completed catalyst was obtained in the same manner as in Example 1, except that 75 g of the alumina-modified cerium oxide obtained above was used instead of the commercially available cerium oxide powder. When the coating layer of this catalyst was analyzed by EPMA, it was found that both Pt-containing zirconia and Rh-containing zirconia were dispersed with an average particle size of 6 μm. Each catalyst contained 0.065 g of Pt and 0.013 g of Rh. Example 9 Alumina sol (containing 10% by weight alumina) 94.7g
was mixed with 340 g of cerium carbonate (Ce content: 47% by weight as CeO ₂ ) and 100 ml of water, dried at 130°C for 5 hours, and then calcined in air at 500°C for 1 hour to form alumina-modified cerium oxide (CeO2). /Al=5 atomic ratio)
was prepared. A completed catalyst was obtained in the same manner as in Example 1, except that 75 g of the alumina-modified cerium oxide obtained above was used instead of the commercially available cerium oxide powder. When the coating layer of this catalyst was analyzed by EPMA, it was found that Pt-containing zirconia and Rh-containing zirconia were both dispersed with an average particle size of 1 μm. Each catalyst contained 0.065 g of Pt and 0.013 g of Rh. Comparative Example 1 An aqueous slurry for coating was prepared by wet-pulverizing 150 g of activated alumina with a specific surface area of 100 m ² /g used in Example 1 and 75 g of commercially available cerium oxide powder in a ball mill for 20 hours. This coating aqueous slurry was coated on a cordierite monolithic carrier in the same manner as in Example 1. The carrier coated with activated alumina and cerium oxide was immersed in a mixed solution of dinitrodiamine platinum nitric acid aqueous solution and rhodium nitrate aqueous solution, pulled out, excess aqueous solution removed with compressed air, and heated at 130°C for 3 hours. The catalyst was dried and calcined in air at 400°C for 2 hours to obtain a finished catalyst. When the coating layer of this catalyst was analyzed by EPMA, neither Pt nor Rh particles were detected as particles larger than 0.5μ. Each catalyst contained 0.065 g of Pt and 0.013 g of Rh. Comparative Example 2 Specific surface area 60 m ² /g, average particle size used in Example 1
An aqueous slurry for coating was prepared by wet-pulverizing 150 g of 200 Å zirconia and 75 g of commercially available cerium oxide powder in a ball mill for 20 hours. This aqueous slurry for coating was prepared in Example 1.
A cordierite monolithic support was coated in the same manner as described above. Pt and Rh were supported on this carrier coated with zirconia and cerium oxide in the same manner as in Comparative Example 1 to obtain a completed catalyst. When the coating layer of this catalyst was analyzed by EPMA, neither Pt nor Rh particles were detected as particles larger than 0.5μ. Each catalyst contained 0.065 g of Pt and 0.013 g of Rh. [Effect of the invention] Catalysts from Example 1 to Example 9 and Comparative Example 1
The catalyst performance of the catalysts from Comparative Example 2 to Comparative Example 2 was investigated after aging in an electric furnace. Electric furnace aging was carried out in a very harsh high-temperature oxidizing atmosphere by heating the catalyst at 900°C in air for 20 hours. The evaluation of catalyst performance after aging was conducted using a commercially available electronically controlled engine (4 cylinders 1800c.c.).
A multi-converter filled with each catalyst was connected to the engine's exhaust system. The three-way performance of the catalyst was evaluated under the conditions of a catalyst inlet gas temperature of 450°C and a space velocity of 90,000 hr ^-1 . At this time, a 1Hz sine wave signal is introduced from an external oscillator to the engine control unit to adjust the air-fuel ratio (A/F) by ±0.5A/F.
F, the average air-fuel ratio was continuously changed while vibrating at 1 Hz, and the catalyst inlet and outlet gas compositions at this time were analyzed simultaneously, and the average air-fuel ratio was determined to be from A/F = 15.1.
The purification rates of CO, HC and NO up to 14.1 were determined. Plot the CO, HC, and NO purification rates vs. inlet air-fuel ratio obtained as above on a graph to create a ternary characteristic curve, and then Purification rate and
The HC purification rate at the A/F value at the intersection was determined and used as a standard for evaluating the three-way performance of the catalyst. In addition, the purification performance of the catalyst at low temperatures varies by ±
The engine was operated with the average air-fuel ratio fixed at A/F = 14.6 while vibrating at 0.5 A/F and 1 Hz. A heat exchanger was installed in front of the catalytic converter in the engine exhaust system, and the catalyst inlet gas was Temperature 200℃
We analyzed the gas composition at the catalyst inlet and outlet when continuously changing the temperature from
Evaluation was made by determining the NO purification rate. Plot the CO, HC and NO purification rates versus catalyst inlet gas temperature obtained as above on a graph,
The catalyst inlet gas temperature (T ₅₀ ) at which the purification rate was 50% was determined and used as a standard for evaluating the purification performance of the catalyst at low temperatures. Table 1 shows the results obtained by the above catalyst performance evaluation method.

[Table] Next, the catalytic activities of the catalysts of Examples 1 to 9 and the catalysts of Comparative Examples 1 to 2 after engine endurance running were investigated. Commercially available electronically controlled engine (8 cylinder 4400
cc), and a multi-converter filled with each catalyst was connected to the engine's exhaust system for durability testing. The engine is operated in a mode of steady operation for 60 seconds and deceleration for 6 seconds (during deceleration, fuel is cut off and the catalyst is exposed to severe conditions of high temperature oxidizing atmosphere), and the catalyst inlet gas temperature is 80°C during steady operation.
The catalyst was aged for 50 hours under the following conditions. Catalyst performance evaluation after engine endurance running was performed in exactly the same manner as the evaluation after electric furnace aging, and three-way performance of the catalyst and purification performance at low temperatures were compared. The results are shown in Table 2.

【table】

[Table] As is clear from Tables 1 and 2, zirconia carrying platinum, palladium, and rhodium disclosed in the present invention at a high loading rate is coated as agglomerated particles having an average particle diameter in the range of 0.5 to 20μ. The catalysts of Examples 1 to 9, which were dispersed in a layer, all exhibited significantly superior catalytic performance than the catalysts of Comparative Examples 1 and 2, in which precious metals were supported in a conventional manner. Moreover, the catalysts of Examples 7 to 9, which used alumina-modified cerium oxide as the cerium oxide, exhibited even higher performance. From the above results, it is clear that the catalyst disclosed in the present invention has excellent durability with little deterioration even under severe conditions such as high-temperature oxidizing atmosphere as well as under normal engine running conditions.

Claims (1)

[Scope of Claims] 1 Agglomerated particles of zirconia (a) having an average particle size of 0.5 to 20μ and carrying platinum and/or palladium in a range of 5 to 30% by weight and rhodium in a range of 1 to 20% by weight. A catalyst composition containing cerium oxide (b) and optionally a refractory inorganic oxide (c) is coated and supported on a honeycomb carrier having an integral structure; A catalyst for exhaust gas purification, characterized in that zirconia (a) has a specific surface area of at least 10 m ² /g and the average particle size of its primary particles is 2000 Å or less. 2 1 of the zirconia (a) per 1 of the carrier
~20g of the cerium oxide (b) as CeO ₂
2. The catalyst according to claim 1, wherein the catalyst supports 150 g of the refractory inorganic oxide (c) and, if necessary, 50 to 200 g of the refractory inorganic oxide (c). 3. The cerium oxide (b) contains a water-insoluble cerium compound, at least one selected from the group consisting of a water-soluble aluminum compound and an alumina hydrate.
3. The catalyst according to claim 1 or 2, wherein the catalyst is an alumina-modified cerium oxide obtained by impregnating and firing seeds. 4. The water-insoluble cerium compound is selected from the group consisting of cerium carbonate, cerium oxide, and cerium hydroxide, and the water-soluble aluminum compound is selected from the group consisting of aluminum nitrate, aluminum chloride, and aluminum sulfate. 4. The catalyst according to claim 3, wherein the catalyst is 5 The composition of the alumina-modified cerium oxide is the atomic ratio of cerium to aluminum.
The catalyst according to claim 3 or 4, characterized in that the Ce/Al ratio is 1 to 20.