JPH0547263B2 - - 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 purification catalyst 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. In particular, it relates to an exhaust gas purifying catalyst that has excellent durability even when used under harsh conditions such as high-temperature oxidizing atmospheres, and has a high purification ability at low temperatures for the above-mentioned harmful components. . [Prior art] Conventionally, in exhaust gas purification catalysts, noble metals are supported on a refractory inorganic oxide with a high surface area such as activated alumina in a dispersed manner as much as possible, and rare earth elements, zirconia, alkaline earth metals, etc. Efforts have been made to improve or maintain the durability of catalysts by adding them to catalyst compositions. However, catalysts with highly dispersed noble metals supported on high surface area alumina etc. have high initial activity, but when exposed to harsh conditions such as high-temperature oxidizing atmospheres, noble metal particles grow and interact with the support material. There was a problem in that it caused a large deterioration in activity. In addition, Japanese Patent Publication No. 57-29215 and Japanese Patent Application Laid-open No. 57-153737 propose a method in which a coating layer containing alumina and zirconia is formed on a support and then a precious metal is supported, but in both cases, the precious metal is Most or a part of it is substantially highly dispersedly supported on alumina, resulting in the same results as above, and sufficient effects are not expected. Moreover, when cerium oxide is present in the coating layer, it is clear that rhodium supported on the cerium oxide is deactivated when exposed to severe conditions such as a high temperature oxidizing atmosphere due to the interaction between the rhodium and the cerium oxide. Also, zirconia (US Pat. No. 4,233,189) is used as a carrier material that does not interact with noble metals, especially rhodium.
or alpha alumina (U.S. Patent No. 4172047)
have been proposed in this field. However, it has been pointed out that zirconia and alpha alumina usually have a low surface area, and when rhodium is supported on them, the initial activity is poor and the purification ability at low temperatures after durability is not sufficient. [Means for Solving the Problems] As a result of intensive research, the present inventors have found that by supporting a noble metal with good dispersion on fine-grained zirconia with a high surface area, it has high initial activity at high and low temperatures. The precious metal is stably supported on the zirconia, and by interposing the refractory inorganic oxide and rare earth oxide between the precious metal-supported zirconia particles, the particle growth of the zirconia fine particles themselves is suppressed, and even in harsh conditions such as high-temperature oxidizing atmospheres. They discovered that it has excellent durability even when used under various conditions, leading to the completion of the present invention. That is, the present invention is specified as follows. (1) Rhodium or a platinum group metal containing rhodium and platinum and/or palladium is deposited on zirconia powder having a specific surface area of at least 10 m ² /g and primary particles having an average particle size of 2000 Å or less. An exhaust gas comprising a honeycomb carrier having an integral structure coated with a catalyst composition containing platinum group metal-supported zirconia and also containing a refractory inorganic oxide and a rare earth oxide. Catalyst for gas purification. (2) 0.5% of a platinum group metal containing rhodium or rhodium and platinum and/or palladium;
The catalyst according to (1) above, wherein the catalyst is supported on zirconia powder so as to have a composition in the range of 30% by weight. (3) The content of the zirconia powder in the catalyst composition
The catalyst according to (1) or (2) above, characterized in that the content is in the range of 0.5 to 50% by weight. (4) The catalyst according to (1) above, wherein the refractory inorganic oxide is activated alumina. (5) The catalyst according to (1) above, wherein the rare earth oxide is cerium oxide. [Function] The catalyst composition in the present invention is composed of zirconia supporting a platinum group metal containing rhodium, an inorganic refractory oxide, and a rare earth oxide. The zirconia used in the present invention is at least 10
m ² /g, preferably 60 to 100 m ² /g, and the primary particles are 2000 Å or less, preferably 500 Å or less
It has an average particle size of Å or less, and 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, etc. be. The amount of zirconia used can range from 0.5 to 50% by weight of the catalyst composition, but sufficient effects of the invention can be exhibited with an amount of 0.5 to 10% by weight. If the amount of zirconia used exceeds 50% by weight of the catalyst composition, particle growth among zirconia particles will be promoted, causing a decrease in activity. The platinum group metal supported on zirconia requires rhodium, and by coexisting with platinum or palladium, the low-temperature activity after high-temperature durability is further improved. It is preferable that the total weight ratio of platinum and palladium coexisting with rhodium is 1/5 to 5.
It is supported in a range of 0.5 to 30% by weight. Note that the entire amount of platinum group metals does not need to be supported on zirconia, and some platinum group metals may be supported on either inorganic oxides such as alumina or rare earth oxides such as cerium oxide or lanthanum oxide.
It may be contained alone in the catalyst composition as a noble metal black such as platinum black. The method of supporting noble metals on zirconia is a conventional method and is not well specified.
Rhodium sources include rhodium chloride, rhodium nitrate, rhodium sulfate, etc. Platinum or palladium sources include chloroplatinic acid, dinitrodiammine platinum, palladium chloride, palladium nitrate, etc., and two or more types can be used in the form of an aqueous solution or an alcoholic solution. When supporting platinum group metals on zirconia, they may be impregnated separately or simultaneously. Thereafter, it is supported on zirconia by drying and firing. In addition to the above-mentioned zirconia, examples of the refractory inorganic oxide used in the present invention include alumina, silica, titania, magnesia, etc., and use of alumina, particularly activated alumina, is preferred. Any of γ, δ, θ, α, χ, κ, and η crystal forms of alumina can be used. The refractory inorganic oxide may be contained in the catalyst composition as it is, but rare earth metals and base metal elements such as chromium, manganese, iron, cobalt, nickel, and zirconium may be added to alumina etc. in the form of oxides of 0.1 to 30% by weight. Further improvement in purification ability can be seen by carrying %. Examples of rare earth oxides include oxides of cerium, lanthanum, neodymium, etc., and use of cerium oxide is particularly preferred. As mentioned above, rare earth oxides may be supported in a range of 0.1 to 30% by weight on a refractory non-oxide such as alumina, but they may be supported in the form of oxides, carbonates, hydroxides, etc. during firing or use. An oxide in the form of an oxide may be included in the catalyst composition. In the latter case, it can be contained in the catalyst composition in an amount of 5 to 80% as an oxide. In the present invention, platinum group metals, especially rhodium-containing platinum group metals, are stably supported on zirconia having a high surface area and consisting of extremely fine particles, so that they can be easily bonded to support materials, rare earth oxides, base metal oxides, etc. It has become possible to contain larger amounts of rare earth oxides and base metal oxides in the catalyst composition than before by suppressing the adverse effects caused by interactions, and it has become possible to improve the durability and purification ability of the catalyst as described above. The thus obtained platinum group metals, especially rhodium-containing platinum group metal-supported zirconia, rare earth oxides, and refractory inorganic oxides are made into an aqueous slurry using a ball mill, etc., and coated in water on a honeycomb carrier having an integral structure, and then dried. Then, if necessary, it is calcined to obtain a finished catalyst. The honeycomb carrier having an integral structure used in the present invention refers to a ceramic carrier such as cordierite, mullite, α-alumina, etc., and a metal monolith carrier such as stainless steel or Fe-Cr-Al alloy. 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 10.0 g of zirconia (manufactured by Daiichi Kigenso Kagaku) having a specific surface of 70 m ² /g and an average particle size of 200 Å was added to rhodium.
Impregnated with rhodium chloride aqueous solution containing 0.3g
It was dried at 120°C for 12 hours. Then 1 in air at 500℃
A zirconia powder containing 2.9% by weight of Rh was prepared by firing for a period of time. Next, a chloroplatinic acid aqueous solution containing 1.5 g of platinum was added to 150 g of activated alumina with a specific surface area of 150 m ² /g.
impregnated with water, dried at 120℃ for 12 hours, and then exposed to air.
Pt-containing alumina powder was obtained by firing at 500°C for 1 hour. An aqueous slurry was prepared by wet-pulverizing the two powders thus obtained and 75 g of commercially available cerium oxide powder in a ball mill for 20 hours. A cordierite monolithic carrier with an outer diameter of 33 mmφ and a length of 76 mm L having approximately 400 gas flow cells per square inch of cross-sectional area was immersed in the above slurry, taken out, and then the excess slurry in the cells was blown with compressed air. Catalyst A was obtained by drying at ℃ for 3 hours. As a result of measuring with fluorescent X-rays, catalyst A is
Each catalyst contained 0.056 g of Pt and 0.011 g of Rh. Example 2 Zirconia powder similar to that used in Example 1
15.0g was impregnated in a mixture of a rhodium chloride aqueous solution containing 0.3g of rhodium and a chloroplatinic acid aqueous solution containing 1.5g of platinum, dried at 120°C for 12 hours, and then calcined in air at 500°C for 1 hour to give a weight of 1.8g. A zirconia powder containing % Rh and 8.9 wt % Pt was prepared.
The above powder, 145 g of activated alumina similar to those used in Example 1, and 75 g of cerium oxide were wet-milled in a ball mill for 20 hours to prepare an aqueous slurry, and catalyst B was obtained in the same manner as in Example 1. Catalyst B is
Each catalyst contained 0.052 g of Pt and 0.010 g of Rh. Example 3 In Example 2, instead of activated alumina, a powder (iron-containing activated alumina) obtained by dissolving 25.3 g of ferric nitrate in 100 g of pure water, impregnating 140 g of activated alumina, and then drying and firing it was used. Catalyst C was obtained in the same manner as in Example 2, except that the catalyst C was used. Catalyst C is
Each catalyst contained 0.054 g of Pt and 0.011 g of Rh. Comparative Example 1 Activated alumina similar to that used in Example 1
An aqueous slurry was prepared by wet-pulverizing 160 g of cerium oxide and 75 g of cerium oxide for 20 hours in a ball mill, and water-coated onto a cordierite monolith carrier in the same manner as in Example 1. The slurry was then dried at 140°C for 3 hours, and then heated at 500°C in air. It was baked for 1 hour. The monolithic support thus treated was immersed in a mixed aqueous solution of chloroplatinic acid and rhodium chloride, dried, and then exposed to air.
A catalyst was obtained by calcining at 400°C for 1 hour. The catalyst is
Each catalyst contained 0.055 g of Pt and 0.011 g of Rh. Comparative Example 2 Activated alumina similar to that used in Example 1
An aqueous slurry was prepared by wet-pulverizing 120 g of commercially available zirconia powder in a ball mill for 20 hours, and was wash coated on a cordierite monolith support in the same manner as in Example 1, dried at 140°C for 3 hours, and then dried in air. It was baked at 500°C for 1 hour. The monolithic support thus treated is immersed in a mixed aqueous solution of chloroplatinic acid and rhodium chloride,
After drying, the mixture was calcined in air at 400°C for 1 hour to obtain a catalyst. The catalyst is Pt0.056g and Rh0.011g as catalyst 1.
Contains per piece. Example 4 A rhodium nitrate aqueous solution containing ^0.35 g of rhodium and a palladium nitrate aqueous solution containing 3.15 g of palladium were used. A zirconia powder containing 2.3% Rh and 20.3% Pd by weight was prepared by impregnating it with the mixed solution, drying it at 120°C for 12 hours, and then calcining it in air at 500°C for 1 hour. Next, 56.1 g of cerium nitrate and 32.2 g of ferric nitrate were dissolved in 200 g of pure water, mixed with 200 g of activated alumina with a specific surface area of 100 m ² /g, dried at 120°C for 12 hours, and then calcined in air at 700°C for 1 hour. _{CeO2 and Fe2O2}
An alumina-containing powder was obtained. Obtained in this way 2
An aqueous slurry was prepared by wet milling different powders in a ball mill for 20 hours. The obtained slurry was wash-coated onto a monolithic cordierite carrier in the same manner as in Example 1, and then dried at 140°C for 3 hours to obtain catalyst D. Catalyst D is Pd0.120
g and Rh0.013 g per catalyst. Example 5 Zirconia 12.0 similar to that used in Example 4
impregnated in a mixed solution of a rhodium nitrate aqueous solution containing 0.35 g of rhodium and a palladium nitrate aqueous solution containing 0.35 g of palladium, dried at 120°C for 12 hours, and then calcined in air at 500°C for 1 hour.
A zirconia powder containing wt% Rh and 2.8 wt% Pd was prepared. Next, an aqueous solution of 56.1 g of cerium nitrate and 32.2 g of ferric nitrate dissolved in 200 g of pure water and palladium were added.
A mixed solution of an aqueous palladium nitrate solution containing 2.8 g was mixed with 200 g of activated alumina having a specific surface area of 100 m ² /g, dried at 120°C for 12 hours, and then calcined in air at 600°C for 1 hour. The two types of powder thus obtained were wet-milled in a ball mill for 20 hours to prepare an aqueous slurry, and catalyst E was obtained in the same manner as in Example 1. Catalyst E contained 0.121 g of Pd and 0.013 g of Rh per catalyst. Example 6 2.3% by weight Rh obtained in the same manner as in Example 4 and
Zirconia powder containing 20.3% Pd and specific surface area 90
m ² /g activated alumina 150g and cerium oxide 80g
Aqueous slurry was prepared by wet-pulverizing g in a ball mill for 20 hours, and catalyst F was obtained in the same manner as in Example 1. Catalyst F contained 0.115 g of Pd and 0.012 g of Rh per catalyst. Comparative Example 3 56.1 g of cerium nitrate and 32.2 g of ferric nitrate were dissolved in 200 g of pure water, mixed with 200 g of activated alumina with a specific surface area of 100 m ² /g, dried at 120°C for 12 hours, and then dried in air at 700°C. It was baked for 1 hour. An aqueous slurry was prepared by wet-pulverizing the above powder in a ball mill for 20 hours, and the slurry was wash-coated onto a cordierite monolith carrier in the same manner as in Example 1.
It was dried at 140°C for 3 hours and then fired in air at 500°C for 1 hour. The monolithic support thus treated was immersed in a mixed aqueous solution of palladium chloride and rhodium chloride, dried, and then calcined in air at 400° C. for 1 hour to obtain a catalyst. Catalyst is Pd0.123g and Rh0.013g
per catalyst. Comparative Example 4 Commercially available zirconia was fired at 1000° C. for 10 hours to obtain powder with a specific surface area of 5 m ² /g and an average particle size of 5000 Å.
A catalyst was obtained in the same manner as in Example 4 except that the above zirconia was used. The catalyst contained 0.120 g Pd and 0.013 g Rh per catalyst. [Effects of the Invention] The catalyst performance of the catalysts of Examples 1 to 3 and the catalysts of Comparative Examples 1 and 2 was investigated when they were new and after aging in an electric furnace. Electric furnace aging in air 900
The experiment was carried out in a very harsh high-temperature oxidizing atmosphere at ℃ for 20 hours. The catalyst performance was evaluated using an electronically controlled engine (4
cylinder (1800c.c.) to control the catalyst inlet gas temperature.
The purification efficiency of CO, HC and NOx was investigated by continuously changing the temperature from 200°C to 450°C using a heat exchanger.
The space velocity (SV) at this time is 90000hr ^-1 ,
Set the average air-fuel ratio to A/F = 14.6, ±0.5A/F,
The engine was operated while vibrating at 1Hz. C.O.
Table 1 shows the catalyst inlet gas temperature (T50) at which the purification rate of HC and NOx becomes 50%. In addition, the catalyst performance of the catalysts of Examples 1 to 3 and the catalysts of Comparative Examples 1 and 2 after running the engine for durability was investigated. The durable engine uses an electronically controlled engine (8 cylinders 4400 c.c.) and operates 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 a high temperature oxidizing atmosphere). The catalyst was aged for 50 hours under conditions such that the catalyst temperature was 850°C during steady operation. Catalyst performance evaluation after engine endurance running was conducted in the same manner as described above, and the results are shown in Table 2. Next, the catalyst performance of the catalysts of Examples 4 to 6 and Comparative Examples 3 to 4 was examined when they were new and after 50 hours of engine durability. The endurance engine used was an electronically controlled engine (6 cylinders 2400 c.c.).
The durability conditions are that secondary air is introduced and the air-fuel ratio is A/F.
= 14.5 to 17.5, the mode durability is carried out in which oxygen deficient atmosphere (rich) and oxygen rich atmosphere (lean) are repeated every 30 seconds, and the catalyst temperature is up to 950℃
had reached. For evaluation of catalyst performance, the purification rate of HC, CO, and NO was examined using the same engine used for endurance testing with A/F = 14.6 and SV = approximately 140,000 hr ^-1 . When new, the evaluation is performed at a catalyst inlet temperature of 500℃, and after durability, evaluation is performed at a catalyst inlet temperature of 500℃ and 700℃.
The results are shown in Tables 3 and 4. From the above results, Examples 1 to 3 and 4 in which platinum group metals were supported on zirconia with a high surface area and fine particles.
It is clear that the catalysts No. 6 to 6 have excellent initial performance and excellent durability even when exposed to severe durability conditions such as high temperature oxidizing atmosphere.

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Claims (1)

[Claims] 1. Rhodium or a platinum group metal containing rhodium and platinum and/or palladium, which has a specific surface area of at least 10 m ² /g and whose primary particles have an average particle size of 2000 Å or less A honeycomb carrier having an integral structure is coated with a catalyst composition containing platinum group metal-supported zirconia supported on zirconia powder, and also containing a refractory inorganic oxide and a rare earth oxide. Characteristic exhaust gas purification catalyst. 2 Rhodium or a platinum group metal containing rhodium and platinum and/or palladium from 0.5 to
Claim 1, characterized in that the zirconia powder is supported on the zirconia powder so as to have a composition in the range of 30% by weight.
Catalysts as described. 3 The content of the zirconia powder in the catalyst composition is
Catalyst according to claim 1 or 2, characterized in that the content is in the range of 0.5 to 50% by weight. 4. The catalyst according to claim 1, wherein the refractory inorganic oxide is activated alumina. 5. The catalyst according to claim 1, wherein the rare earth oxide is cerium oxide.