Use of a catalyst for purification of exhaust gas from Diesel engine

Abstract: A catalyst for purification of exhaust gas from Diesel engine wherein catalyst component containing (a) refractory inorganic oxide, (b) oxide of at least one element selected from the group consisting of praseodym­ ium, neodymium and samarium, and (c) at lest one noble metal selected from the group consisting of platinum, palladium and rhodium provided that palladium is never selected alone is borne on refractory three-dimensional structure.

Description: This invention relates to the use of a catalyst for purifying harmful components in exhaust gases from Diesel engines whereby to enable the removal of fine particle substances (mainly consisting of liquid or solid high molecular weight hydrocarbon fine particles and solid carbon fine particles) from exhaust gases from Diesel engines at a low temperature, the catalyst having high durability under severe conditions such as a high temperature oxidation atmosphere. Recently, fine particle substances in exhaust gases from Diesel engines (mainly consisting of solid carbon fine particles, sulfurous fine particles such as sulfate, liquid or solid high molecular weight hydrocarbon fine particles, and the like) have become a problem as regards the environment and hygiene. Since almost all these fine particles have a particle size of 1 µm or less they readily float into the atmosphere and they can be taken into the human body by respiration. Therefore, governmental regulations on the removal of these fine particles from the exhaust gases of Diesel engines are becoming stricter. For removal of these fine particles, there has been studied the use of a catalyst substance capable of burning carbonaceous fine particles deposited on a refractory three-dimensional structure such as ceramic foam, wire mesh, metal foam or wall flow type ceramic honeycomb. In this way fine particle substances contained in exhaust gases from Diesel engines are captured and are burned for purification under the discharge conditions (gas composition and temperature) of an exhaust gas generated in ordinary operation of the engine (or using heating means such as electric heater). A catalyst for use in the purification of exhaust gases from Diesel engines should have a high performance in burning carbonaceous fine particles, should be capable of igniting them even at a fairly low temperature and should withstand continuous operation under a high load (i.e. should have a high temperature durability). However, no catalyst which sufficiently meets these criteria has so far been proposed. Proposals which have been made for the purpose of enhancing the purification efficiency of the carbonaceous fine particles are as follows. Catalysts for the combustion of carbonaceous fine particles using a platinum group metal disclosed as catalysts effective for SOF (soluble organic fraction, i.e. organic solvent-soluble components) in Japanese Laid-Open Patent Publication No. 24597/1980 include rhodium (7.5%)/platinum alloy, platinum/palladium (50/50) mixture, palladium deposited on tantalum oxide or cerium oxide, and an alloy consisting of palladium and 75% by weight or less of platinum. In addition, Japanese Laid-Open Patent Publications Nos. 129030/1986, 149222/1986 and 146314/1986 disclose catalyst compositions comprising palladium and rhodium as main components, and alkali metal, alkaline earth metal, copper, lanthanum, zinc or manganese as minor components. Further, Japanese Laid-Open Patent Publication No. 82944/1984 discloses a catalyst composition comprising at least one member selected from copper, alkali metal, molybdenum and vanadium, and at least one member selected from platinum, rhodium and palladium. Recently, following improvements in Diesel engines, there is a tendency for the content of fine particle substances in exhaust gases from Diesel engines to decrease and there is a tendency for the SOF content of the fine particle substances to increase. Further, it has been planned to reduce the S content of fuels. Thus, exhaust gases from the latest Diesel engines are different from the exhaust gases from earlier Diesel engines in properties. However, a catalyst for use in the purification of exhaust gases fit for such latest circumstances has so far not been proposed. The present invention seeks to provide the use of a catalyst in the purification of exhaust gases of Diesel engines which can purify even at low temperatures harmful components in Diesel engine exhaust gases such as carbonaceous fine particles, hydrocarbons and carbon monoxide, and which is capable of maintaining itself stably for a long time under the exhaust gas atmosphere of Diesel engines (namely has heat resistance and durability). According to the present invention there is provided the use in the purification of an exhaust gas from a Diesel engine of a catalyst which comprises catalyst components containing (a) 3 to 300 g of a refractory inorganic oxide, (b) 2 to 200 g of at least one element selected from praseodymium, neodymium and samarium, (c) at least 0.1 g of at least one noble metal selected from platinum and rhodium and optionally also palladium, the total amount of noble metal being from 0.1 to 10 g, and optionally (d) 0.1 to 3 g of an oxide of at least one element selected from copper, silver zinc, potassium and sodium, the catalyst components being deposited on a refractory three-dimensional structure and each specified amount being per litre of said refractory three-dimensional structure. The reason why palladium is not selected alone as the noble metal (c) in accordance with the present invention is that a prior application by the same Applicant, namely European Patent Application No. 88118309.9 (EP-A-315896) (which is prior art only by virtue of Article 54(3) and (4) EPC), discloses and claims an exhaust gas purification catalyst for purifying carbonaceous fine particles comprising a refractory three-dimensional structure having deposited thereon (a) a refractory inorganic oxide, (b) palladium and (c) an oxide of at least one element selected from praseodymium, neodymium and samarium as catalyst components. The oxides of rare earth elements used as catalyst component (b) exhibit a high activity in an oxygen exchange reaction in an oxidation atmosphere and function as an oxygen supply source for combustion of carbonaceous fine particles, and as a result display an excellent purification performance even at a low temperature by a synergistic effect with the noble metal (c). The oxides (b) also function to restrain deterioration of the activity of the noble metal (c) under the atmosphere of Diesel engine exhaust gas. Therefore, the use of a catalyst in accordance with the present invention makes possible an excellent performance for a long time under the atmosphere of Diesel engine exhaust gas at a high temperature of 400°C or more. Examples of the above refractory inorganic oxide to be used as catalyst component (a) include activated alumina, silica, titania, zirconia, silica-alumina, alumina-zirconia, alumina-titania, silica-titania, silica-zirconia, titania-zirconia and zeolite, and zirconia is mentioned among them as a base material suitable for enhancement of heat stability of the rare earth element. Suitably used as starting materials of praseodymium, neodymium and samarium are their commercially available oxides, chlorides, nitrates or carbonates. There can be used as starting materials of platinum chloroplatinic acid, dinitrodiaminoplatinum, platinumtetramine chloride or platinumsulfito complex salt, and as starting materials of rhodium rhodium nitrate, hexamminerhodium chloride or rhodiumsulfito complex salt. The amount of the refractory inorganic oxide (a) to be deposited is 3 to 300 g, preferably 10 to 200 g per litre of the three-dimensional structure. The amount of the oxide of at least one element of praseodymium, neodymium and samarium (b) to be deposited is 2 to 200 g, preferably 3 to 100 g in total per litre of the three-dimensional structure. The amount of the at least one noble metal (c) to be deposited is 0.1 to 10 g, preferably 0.5 to 5 g, in total per litre of the three-dimensional structure. The catalyst to be used in accordance with the invention may contain as additional component (d) at least one element selected from copper, silver, zinc, potassium and sodium in an amount of 0.1 to 3 g, preferably 0.2 to 1 g, in total per litre of the three-dimensional structure in order to suppress generation of sulfates [sulfur trioxide (SO₃) and sulfuric acid mist generated by oxidation of sulfur dioxide (SO₂)]. However, when a fuel containing 0.1% by weight or more of sulfur content is used in a Diesel engine installed with such a catalyst which contains platinum, a large amount of sulfates are generated, and thus use of platinum causing an increase of sulfurous fine particles is not desirable, but it is preferred to use a catalyst wherein palladium and rhodium are deposited in an amount in the range of 0.1 to 10 g and 0.01 to 1.0 g respectively per liter of the refractory three-dimensional structure, and the depositing ratio of palladium/rhodium is 1 to 50. Although the process of preparation of the catalyst of the invention is not limited, the following processes are mentioned as preferred ones: (1) Powder of refractory inorganic oxide is wet-pulverized to form a slurry, a refractory three-dimensional structure is dipped in the slurry, excessive slurry is removed, and the resulting structure is dried at 80 to 250 °C and then calcined at 300 to 850 °C, preferably 400 to 800 °C to obtain the three-dimensional structure coated with the refractory inorganic oxide. The structure coated with the refractory inorganic oxide is then dipped in an aqueous solution containing, each in a predetermined amount, palladium compound; compound of at least one element selected from praseodymium, neodymium and samarium; and compound of at least one element selected from platinum, palladium and rhodium, and, after removal of excessive solution, dried at 80 to 250 °C and then calcined at 300 to 850 °C, preferably 400 to 800 °C to obtain a completed catalyst. (2) A refractory three-dimensional structure is dipped in a slurry containing oxide of at least one element selected from praseodymium, neodymium and samarium; and refractory inorganic oxide, and, after removal of excessive slurry, dried at 80 to 250 °C and then calcined at 300 to 850 °C, preferably 400 to 800 °C to provide a coat layer on the refractory three-dimensional structure. The resulting three-dimensional structure is dipped in an aqueous solution containing a predetermied amount of compound of at least one element selected from platinum, palladium and rhodium, dried at 80 to 250°C, and then calcined at 300 to 850°C, preferably 400 to 800 °C to obtain a completed catalyst. (3) Refractory inorganic oxide is dipped in an aqueous solution containing, each in a predetermined amount compound of at least one element selected from praseodymium, neodymium and samarium; and compound of at least one element selected from platinum, palladium and rhodium, dried at 80 to 250 °C, and then calcined at 300 to 850 °C, preferably 400 to 800 °C to deposit and fix the oxide of the element on the refractory inorganic oxide. The resulting refractory inorganic oxide is then wet-pulverized to form a slurry. A refractory three-dimensional structure is dipped in this slurry, and, after removal of excessive slurry, dried at 80 to 250 °C and then calcined at 300 to 850 °C, preferably 400 to 800 °C to obtain a completed catalyst. Examples of the refractory three-dimensional structure to be used include a ceramic foam, an open flow ceramic honeycomb, a wall flow type honeycomb monolith, an open flow metal honeycomb, a metal foam and a metal mesh. In this connection, when exhaust gas from Diesel engines contain 100 mg or less of fine particle substances per m³ of the exhaust gas and the fine particle substances 20 % or more of SOF, an open flow type ceramic honeycomb or a metal honeycomb is preferably used as the refractory three-dimensional structure. The exhaust gas purification catalyst used in accordance with this invention containing the refractory inorganic oxide; the oxide of at least one of praseodymium, neodymium and samarium; and at least one noble metal selected from platinum, palladium and rhodium (provided that palladium is not selected alone) has a combustion performance of carbonaceous fine particles at low temperatures, displays its excellent purification performance in a good reproducibility even at low temperatures, and has excellent heat resistance and durability and thus can maintain its initial activity for a long time in the exhaustion condition (gas composition and temperature) of Diesel engine exhaust gas. The present invention is illustrated in more detail below according to examples and comparative examples. EXAMPLE 1 Alumina (2 kg) having a specific surface area of 130 m²/g was weighed and wet-pulverized with water to form a slurry. Wall flow type monolith carriers made of cordierite (honeycomb type and wall flow type three-dimensional structures wherein each adjacent opening of the both end faces is alternately blocked so that gas is passed only through the partition wall) having a size of 14.4 cm (5.66 inches) in diameter x 15.2 cm (6.00 inches) in length was dipped in the slurry, and, after removal of excessive slurry, dried at 150 °C for 3 hours, and then calcined at 500 °C for one hour to obtain a three-dimensional structure deposited with alumina. Then, 900 g of neodymium nitrate [Nd(NO₃)₃.6H₂O] and chloroplatinic acid in an amount of 9.2 g in terms of platinum were dissolved in deionized water to give 1.5 ℓ of a solution. Three-dimensional structures depositing alumina were dipped in this solution, and, after removal of excessive solution, dried at 150 °C for 3 hours and then calcined at 700 °C for 2 hours to obtain completed catalysts. Amounts of alumina, neodymium oxide (Nd₂O₃) and platinum deposited on the catalysts were 20 g, 30 g and 0.8 g respectively per liter of the structures. EXAMPLE 2 Activated alumina (2 kg) having a specific surface area of 150 m²/g was weighed and wet-pulverized with water to form a slurry. The same three-dimensional structures as used in EXAMPLE 1 were dipped in the slurry, and, after removal of excessive slurry, dried at 200 °C for 3 hours and then calcined at 500 °C for 2 hours to obtain structures deposited with alumina. Then, 307.7 g of praseodymium oxide (Pr₆O₁₁), dinitrodiaminoplatinum in an amount of 7.7 g in terms of platinum and rhodium nitrate in an amount of 4.6 g in terms of rhodium were dissolved in a diluted aqueous nitric acid solution to give 2 ℓ of a solution. The above three-dimensional structures deposited with alumina were dipped in this solution, and, after removal of excessive solution, dried at 150 °C and then calcined at 600 °C for 2 hours to obtain completed catalyst. Amounts of alumina, praseodymium oxide, platinum and rhodium deposited on the resulting catalysts are 10 g, 20 g, 0.5 g and 0.3 g respectively per liter of the structures. EXAMPLE 3 Samarium nitrate [Sm(NO₃)₃.6H₂O] (196 g), 1402 g of neodymium nitrate, chloroplatinic acid in an amount of 1.5 g in terms of platinum, palladium chloride in an amount of 4.6 g in terms of palladium and rhodium chloride in an amount of 4.6 g in terms of rhodium were dissolved in deionized water to give 2 ℓ of a solution. Three-dimensional structures which was deposited with 100 g of alumina per liter of the structures, prepared in the same manner as in EXAMPLE 1 were dipped in this solution, and, after removal of excessive solution, dried at 150 °C for 3 hours and calcined at 800 °C for one hour to obtain completed catalysts. Amounts of alumina, samarium oxide (Sm₂O₃), neodymium oxide, platinum, palladium and rhodium deposited on the catalysts were 100 g, 5.0 g, 35 g, 0.1 g, 0.3 g and 0.3 g respectively per liter of the structures. EXAMPLE 4 Alumina (1 kg) having a specific surface area of 150 m²/g was placed in a solution in deionized water of 310 g of praseodymium nitrate, 306 g of neodymium nitrate, palladium nitrate in an amount of 23.5 g in terms of palladium and rhodium nitrate in an amount of 2.35 g in terms of rhodium, and, after sufficient stirring, dried at 150 °C for 6 hours and then calcined at 500 °C for 2 hours to obtain alumina powder containing praseodymium oxide, neodymium, palladium and rhodium. This powder (1 kg) was wet-pulverized to form a slurry. The same three-dimensional structures as used in EXAMPLE 1 were dipped in the slurry, and, after removal of excessive slurry, dried at 150 °C for 2 hours and then calcined at 400 °C for 2 hours to obtain completed catalysts. Amounts of alumina, praseodymium oxide, neodymium oxide, palladium and rhodium deposited on the obtained catalysts were 85 g, 10 g, 10 g, 2 g and 0.2 g respectively per liter of the structures. EXAMPLE 5 A mixed powder of 250 g of activated alumina having a specific surface area of 150 m²/g and 750 g of neodymium oxide having a specific surface area of 60 m²/g was placed in a solution in deionized water of rhodium nitrate in an amount of 40 g in terms of rhodium, and, after sufficient stirring, dried at 150 °C for 6 hours and then calcined at 800 °C for one hour to obtain alumina-neodymium oxide mixed powder containing rhodium. This powder (1 kg) was wet-pulverized to form a slurry, and three-dimensional structures were dipped in the slurry in the same manner as in EXAMPLE 1, and after removal of excessive slurry, dried at 150 °C for 2 hours and calcined at 400 °C for one hour to obtain completed catalysts. Amounts of alumina, neodymium oxide and rhodium deposited on the obtained catalysts were 20 g, 60 g and 3.2 g respectively. EXAMPLE 6 Zirconia powder (1 kg) having a specific surface area of 110 m²/g was placed in a solution in deionized water of 790 g of praseodymium nitrate, 255 g of samarium nitrate, palladium nitrate in an amount of 30 g in terms of palladium and rhodium nitrate in an amount of 1 g in terms of rhodium, and, after sufficient stirring, dried at 150 °C for 6 hours and then calcined at 600 °C for 2 hours to obtain zirconia powder containing praseodymium oxide, samarium oxide, palladium and rhodium. The powder (1 kg) was wet-pulverized to form a slurry. The same three-dimensional structures as used in EXAMPLE 1 were dipped in the slurry, and, after removal of excessive slurry, dried at 150 °C for 5 hours and calcined at 700 °C for one hour to obtain completed catalysts. Amounts of zirconia, praseodymium oxide, samarium oxide, palladium and rhodium deposited on the obtained catalysts were 100 g, 30 g, 10 g, 3 g and 0.1 g per liter of the structures. EXAMPLE 7 Zirconia powder (2 kg) having a specific surface area of 130 m²/g was weighed and wet-pulverized with water to form a slurry. The same three-dimensional structures as used in EXAMPLE 1 were dipped in the slurry, and, after removal of excessive slurry, dried at 150 °C for 3 hours and calcined at 500 °C for 2 hours to obtain structures deposited with zirconia. Then, 833 g of praseodymium nitrate [Pr(NO₃)₃.6H₂O) and platinumtetramine chloride in an amount of 4.5 g in terms of platinum were dissolved in deionized water to obtain 2 ℓ of a solution. The three-dimensional structures deposited with zirconia were dipped in this solution, and, after removal of excessive solution, dried at 150 °C for 3 hours and then calcined at 500 °C for 2 hours to obtain completed catalysts. Amounts of zirconia, praseodymium oxide and platinum deposited on the obtained catalysts were 50 g, 20 g and 0.3 g respectively per liter of the structures. EXAMPLE 8 Titania (2 kg) having a specific surface area of 72 m²/g was weighed and wet-pulverized with water. The same three-dimensional structures as used in EXAMPLE 1 were dipped in the slurry, and, after removal of excessive slurry, dried at 150 °C for 3 hours and calcined at 600 °C for 2 hours to obtain structures depositing titania. Then, 784 g of praseodymium chloride (PrCl₃.6H₂O), 387 g of neodymium chloride (NdCl₃.6H₂O), 380 g of samarium chloride (SmCl₃.6H₂O) and chloroplatinic acid in an amount of 10.9 g in terms of platinum were dissolved in deionized water to obtain 2 ℓ of a solution. The three-dimensional structures deposited with titania were dipped in this solution, and, after removal of excessive solution, dried at 150 °C for 3 hours and calcined at 600 °C for 2 hours to obtain completed catalysts. Amounts of titania, praseodymium oxide, neodymium oxide, samarium oxide and platinum deposited on the obtained catalysts were 30 g, 20 g, 10 g, 10 g and 0.6 g respectively per liter of the structures. EXAMPLE 9 Alumina (1 kg) having a specific surface area of 150 m²/g was placed in a solution of 364 g of neodymium carbonate [Nd₂(CO₃)₃.3H₂O] in deionized water, and, after sufficient stirring, dried at 150 °C for 6 hours and calcined at 500 °C for 2 hours to obtain alumina powder containing neodymium oxide. The same three-dimensional structures as used in EXAMPLE 1 were dipped in a slurry obtained by wet-pul verizing 1 kg of the above powder, and, after removal of excessive slurry, dried at 150 °C for 3 hours and calcined at 500 °C for one hour to obtain structures deposited with the neodymium oxide-containing alumina. The structures deposited with the neodymium oxide-containing alumina were dipped in 2.5 ℓ of a nitric acid-acidified aqueous solution of dinitrodiaminoplatinum in an amount of 1.24 g in terms of platinum to adsorb platinum thereon, and, after removal of excessive solution, dried at 150 °C for 3 hours and then calcined at 700 °C for 2 hours to obtain completed catalysts. Amounts of alumina, neodymium oxide and platinum deposited on the obtained catalysts were 25 g, 5 g and 0.5 g per liter of the structures. EXAMPLE 10 The same procedures as in EXAMPLE 6 were conducted except that zirconia was replaced by composite oxide of zirconia-silica (ZrO₂/SiO₂ weight ratio=4/1) having a specific surface area of 135 m²/g, whereby completed catalysts were obtained. Amounts of zirconia-silica, praseodymium oxide, samarium oxide, palladium and rhodium deposited on the obtained catalysts were 100 g, 30 g, 10 g, 3 g and 0.1 g respectively per liter of the structures. EXAMPLE 11 The same procedures as in Example 3 were conducted except that cordierite foam (ceramic foam, bulk density 0.35 g/cm³, porosity 87.5 %, volume 2.5 ℓ) was used in place of wall flow type honeycomb monolith as three-dimensional structure in EXAMPLE 3, whereby a completed catalyst was obtained. Amounts of alumina, samarium oxide, neodymium oxide, platinum, palladium and rhodium deposited on the obtained catalyst were 100 g, 5.0 g, 35 g, 0.1 g, 0.3 g and 0.3 g respectively per liter of the structure. EXAMPLE 12 Alumina (1 kg) having a specific surface area of 150 m²/g was placed in a solution in deionized water of 1320 g of praseodymium nitrate, 637 g of samarium nitrate, 76 g of copper nitrate [Cu(NO₃)₂.3H₂O], 18.5 g of sodium nitrate (NaNO₃), palladium nitrate in an amount of 150 g in terms of palladium and rhodium nitrate in an amount of 10 g in terms of rhodium. After sufficient stirring of the mixture, the alumina was dried at 150 °C for 6 hours, and calcined at 500 °C for one hour to obtain a mixed powder consisting of alumina, praseodymium oxide, samarium oxide, copper oxide (CuO), sodium oxide (Na₂O), palladium and rhodium. The powder (1 kg) was wet-pulverized to form a slurry, and the same three-dimensional structures as used in EXAMPLE 1 were dipped in the slurry, and, after removal of excessive slurry, dried at 120 °C for 3 hours and calcined at 500 °C for one hour to obtain completed catalysts. Amounts of alumina, praseodymium oxide, samarium oxide, copper oxide, Na₂O, palladium and rhodium deposited on the obtained catalysts were 20 g, 10 g, 5 g, 0.5 g, 0.1 g, 3 g and 0.2 g per the structures. EXAMPLE 13 Alumina (2 kg) having a specific surface area of 150 m²/g was weighed and wet-pulverized with water to form a slurry. Cordierite monolith carriers having a size of 14.4 cm (5.66 inches) in diameter x 15.2 cm (6.00 inches) in length and having open flow gas ventilation cells in a number of about 400 per 6.45 cm² (square inch) of cross section were dipped in the above slurry, and, after removal of excessive slurry, dried at 150 °C for 3 hours and then calcined at 400 °C for 2 hours to obtain structures depositing alumina. Then, the structures deposited with alumina were dipped in 2.5 ℓ of a nitric acid-acidified aqueous solution of dinitrodiaminoplatinum in an amount of 1.24 g in terms of platinum for one hour to adsorb platinum thereon, dried at 150 °C for 6 hours and then calcined at 400 °C for one hour to deposit platinum on alumina. The platinum/alumina-depositing structures were dipped in 2 ℓ of a solution in deionized water of 405 g of praseodymium nitrate and 200 g of neodymium nitrate, and, after removal of excessive solution, dried at 150 °C for 3 hours and then calcined at 700 °C for one hour to obtain completed catalysts. Amounts of alumina, praseodymium oxide, neodymium oxide and platinum deposited on the obtained catalysts were 100 g, 10 g, 5 g and 0.5 g per liter of the structures. EXAMPLE 14 Zirconia (1 kg) having a specific surface area of 110 m²/g was placed in a solution in deionized water of 1320 g of praseodymium nitrate, palladium sulfate in an amount of 10 g in terms of palladium and rhodium nitrate in an amount of 1 g in terms of rhodium, and, after sufficient stirring, dried at 180 °C for 3 hours and then calcined at 700 °C for one hour to obtain zirconia-praseodymium oxide-palladium-rhodium mixed powder. The powder (1 kg) was wet-pulverized to form a slurry, and the same three-dimensional structures as used in EXAMPLE 13 were dipped in the slurry, and, after removal of excessive slurry, dried at 150 °C for 6 hours and calcined at 500 °C for one hour to obtain completed catalysts. Amounts of zirconia, praseodymium oxide, palladium and rhodium deposited on the obtained catalysts were 100 g, 50 g, 1 g and 0.1 g per liter of the structures. EXAMPLE 15 Alumina (1 kg) having a specific surface area of 150 m²/g was placed in a solution in deionized water of 132 g of praseodymium nitrate, palladium nitrate in an amount of 20 g in terms of palladium, rhodium nitrate in an amount of 1 g in terms of rhodium and 24 g of copper sulfate, and, after sufficient stirring, dried at 150 °C for 6 hours and then calcined at 500 °C for one hour to obtain mixed powder of alumina, praseodymium oxide, palladium, rhodium and copper oxide. The powder (1 kg) was wet-pulverized to form a slurry, and the same three-dimensional structures as used in EXAMPLE 13 were dipped in the slurry, and, after removal of excessive slurry, dried at 150 °C for 6 hours and calcined at 500 °C for one hour to obtain completed catalysts. Amounts of alumina, praseodymium oxide, palladium, rhodium and copper oxide deposited on the obtained catalysts were 100 g, 5 g, 2 g, 0.1 g and 0.8 g per liter of the structures. EXAMPLE 16 Alumina-depositing three-dimensional structures prepared in the same manner as in EXAMPLE 13 were dipped in 2 ℓ of a nitric acid-acidified aqueous solution containing 307 g of samarium oxide, chloroplatinic acid in an amount of 2.5 g in terms of platinum, rhodium chloride in an amount of 0.37 g in terms of rhodium and 45 g of silver nitrate (AgNO₃), and, after removal of excessive solution, dried at 150 °C for 3 hours and then calcined at 800 °C for one hour to obtain completed catalysts. Amounts of alumina, samarium oxide, platinum, rhodium and silver oxide (Ag₂O) deposited on the obtained catalysts were 50 g, 20 g, 0.5 g, 0.05 g and 2 g respectively per liter of the structures. EXAMPLE 17 Three-dimensional structures deposited with 200 g of alumina per liter of the structures were obtained in the same manner as in EXAMPLE 13. The alumina-depositing three-dimensional structures were then dipped in 2 ℓ of an aqueous solution prepared by dissolving in deionized water 406 g of praseodymium nitrate, palladium chloride in an amount of 46 g in terms of palladium, rhodium chloride in an amount of 2.3 g in terms of rhodium and 34 g of zinc nitrate [Zn(NO₃)₂.6H₂O], and, after removal of excessive solution, dried at 150 °C for 3 hours and then calcined at 500 °C for one hour to obtain completed catalysts. Amounts of alumina, praseodymium oxide, palladium, rhodium and zinc oxide (ZnO) deposited on the obtained catalysts were 200 g, 10 g, 3 g, 0.15 g and 0.6 g per liter of the structures. COMPARATIVE EXAMPLE 1 Alumina-depositing three-dimensional structure prepared in the same manner as in EXAMPLE 2 was dipped in 2.5 ℓ of a nitric acid-acidified aqueous solution of dinitrodiaminoplatinum in an amount of 1.5 g in terms of platinum to adsorb platinum thereon, and, after removal of excessive solution, dried at 150 °C for 3 hours and then calcined at 500 °C for 2 hours to obtain completed catalyst. Amounts of alumina and platinum deposited on the obtained catalyst were 50 g and 0.6 g respectively. COMPARATIVE EXAMPLE 2 Alumina (1 kg) having a specific surface area of 150 m²/g was placed in a solution prepared by dissolving in deionized water chloroplatinic acid in an amount of 10 g in terms of platinum and rhodium chloride in an amount of 6 g in terms of rhodium, and, after sufficient stirring, dried at 150 °C for 6 hours and then calcined at 500 °C for one hour to obtain platinum-rhodium-containing alumina powder. The same three-dimensional structures as used in EXAMPLE 1 were dipped in a slurry formed by wet-pulverizing the above powder, and, after removal of excessive slurry, dried at 150 °C for 6 hours and calcined at 500 °C for one hour to obtain completed catalysts. Amounts of alumina, platinum and rhodium deposited on the obtained catalysts were 50 g, 0.5 g and 0.3 g per liter of the structures. COMPARATIVE EXAMPLE 3 Alumina-depositing three-dimensional structures prepared in the same manner as in EXAMPLE 2 were dipped in 2 ℓ of an aqueous solution of palladium nitrate in an amount of 46 g in terms of palladium and rhodium nitrate in an amount of 3 g in terms of rhodium, and, after removal of excessive solution, dried at 150 °C for 3 hours and calcined at 500 °C for 2 hours to obtain completed catalysts. Amounts of alumina, palladium and rhodium deposited on the obtained catalyst were 50 g, 3 g and 0.2 g per liter of the structures. COMPARATIVE EXAMPLE 4 Alumina (1 kg) having a specific surface area of 150 m²/g was placed in a solution prepared by dissolving in deionized water palladium nitrate in an amount of 20 g in terms of palladium, rhodium nitrate in an amount of 20 g in terms of rhodium and 607 g of copper nitrate, and, after sufficient stirring, dried at 150 °C for 6 hours and then calcined at 500 °C for one hour to obtain palladium-rhodium-copper oxide-containing alumina powder. The same three-dimensional structures as used in EXAMPLE 1 were dipped in a slurry prepared by wet-pulverizing the above powder, and, after removal of excessive slurry, dried at 150 °C for 6 hours and calcined at 500 °C for one hour to obtain completed catalysts. Amounts of alumina, palladium, rhodium and copper oxide deposited on the obtained catalysts were 50 g, 1 g, 1 g and 10 g respectively per liter of the structures. COMPARATIVE EXAMPLE 5 Alumina-depositing three-dimensional structures prepared in the same manner as in EXAMPLE 13 were dipped in 2.5 ℓ of a nitric acid-acidified aqueous solution of dinitrodiaminoplatinum in an amount of 2.5 g in terms of platinum to adsorb platinum thereon, and, after removal of excessive solution, dried at 150 °C for 3 hours and then calcined at 500 °C for 2 hours to obtain completed catalysts. Amounts of alumina and platinum deposited on the obtained catalysts were 100 g and 1 g per liter of the structures. COMPARATIVE EXAMPLE 6 The same procedures as in EXAMPLE 15 were conducted except that praseodymium nitrate and copper nitrate were not used to obtain completed catalysts. Amounts of alumina, palladium and rhodium deposited on the obtained catalysts were 100 g, 2 g and 0.1 g per liter of the structures. There were shown in Table 1 amounts of the respective components (refractory inorganic oxide(s), platinum group element(s), oxide(s) of rare earth element and additional component(s) deposited on the respective catalysts of the above EXAMPLES 1 to 17 and COMPARATIVE EXMAPLES 1 to 6 per liter of the respective three-dimensional structures. TEST EXAMPLE 1 The following evaluation tests were carried out on the catalysts obtained in EXAMPLES 1 to 12 and COMPARATIVE EXAMPLES 1 to 4. There was used as engine a 4-cylindered Diesel engine having a cubic capacity of 2300 cc. As for fuel, light oil having the S content of 0.03 % by weight was used in case of the platinum-containing catalysts of EXAMPLES 1, 2, 3, 7, 8, 9 and 11 and COMPARATIVE EXAMPLES 1 and 2, whereas light oil having the S content of 0.3 % by weight was used in case of the catalysts of EXAMPLES 4, 5, 6, 10 and 12 and COMPARATIVE EXAMPLES 3 and 4. [Initial test] Capture of fine particles was carried out for about 2 hours under the condition of an engine revolution number of 2500 rpm and a torque of 4.0 kg.m. Then, torque was increased every 5 minutes at intervals of 0.5 kg.m, change in pressure loss of the catalyst layer was continuously recorded, and there were sought the temperature (Te) at which pressure increase due to accumulation of fine particles and pressure decrease due to combustion of fine particles become equal, and the temperature (Ti) at which pressure loss drastically drops due to ignition and combustion. Further, conversion (%) of SO₂ to SO₃ was sought at an exhaust gas temperature of 400 °C. SO₂ concentrations of inlet gas and outlet gas were analyzed by a non-dispersion type infrared analyzer (NDIR method), and conversion (%) of SO₂ was calculated according to the following calculation equation: [300 Hours duration test] Catalyst duration test was conducted under full load of an engine revolution number of 2500 rpm and a catalyst inlet temperature of 600 °C, activity after 300 hours was evaluated in the same manner as in initial evaluation, and thereby activity deterioration was determined. Results of the above measurements are shown in Table 2. Initial stage After 300 hours duration Te (°C) Ti (°C) SO₂ conversion (%) (at 400 °C) Te (°C) Ti (°C) SO₂ conversion (%) (at 400 °C) Example 1 328 341 71 336 357 76 Example 2 351 362 58 368 381 56 Example 3 362 389 21 363 387 24 Example 4 376 392 2.8 381 397 2.6 Example 5 379 397 0.0 392 422 0.0 Example 6 358 368 2.1 356 370 2.4 Example 7 341 356 63 342 351 67 Example 8 319 332 68 343 359 63 Example 9 336 343 78 339 351 74 Example 10 348 363 3.3 345 356 2.9 Example 11 356 378 18 360 378 20 Example 12 371 393 0.3 373 391 0.2 Comparative Example 1 390 408 76 442 463 73 Comparative Example 2 416 423 63 478 493 65 Comparative Example 3 431 465 3.7 507 523 2.9 Comparative Example 4 476 498 0.0 521 543 0.0 TEST EXAMPLE 2 The following evaluation tests were conducted on the catalysts obtained in EXAMPLES 13 to 17 and COMPARATIVE EXAMPLES 5 and 6. As engine was used a commercial available super-charging direct injection type Diesel engine equipped with intercooler (4-cylindered engine, 2800 cc). As for fuel, light oil having the S content of 0.03 % by weight was used in case of the platinum-containing catalysts of EXAMPLES 13 and 16 and COMPARATIVE EXAMPLE 5, and light oil having the S content of 0.3 % by weight was used in case of the catalysts of EXAMPLES 14, 15 and 17 and COMPARATIVE EXAMPLE 6. [Initial test] Amounts of fine particle substances exhausted (Part) in the exhaust gases at the catalyst inlet and outlet under the condition of an engine revolution number of 2,000 rpm, a torque of 8 kg.m and a catalyst inlet temperature of 300 °C was measured using the dilution tunnel method, and thereby purification rate (%) of the fine particle substances was determined. Further, analysis of SO₂ in the exhaust gases at the catalyst inlet and outlet and gaseous hydrocarbon (HC) was carried out at the same time and thereby their conversion (conv. (%)) were determined. [300 Hours duration test] Catalyst duration tests were carried out under full load of an engine revolution number of 2500 rpm and a catalyst inlet temperature of 600 °C, and activity after 300 hours was evaluated in the same manner as in the initial evaluation, and thereby activity deterioration was determined. Results of the above determination are shown in Table 3. It is apparent from Table 2 and Table 3 that the catalysts used in accordance with the present invention are Diesel engine exhaust gas purification catalysts which not only have a high purification performance, but also resist deterioration even under severe conditions such as a high temperature oxidation atmosphere and have excellent durability.

Claims: The use in the purification of an exhaust gas from a Diesel engine of a catalyst which comprises catalyst components containing (a) 3 to 300 g of a refractory inorganic oxide, (b) 2 to 200 g of at least one element selected from praseodymium, neodymium and samarium, (c) at least 0.1 g of at least one noble metal selected from platinum and rhodium and optionally also palladium, the total amount of noble metal being from 0.1 to 10 g and optionally (d) 0.1 to 3 g of an oxide of at least one element selected from copper, silver zinc, potassium and sodium, the catalyst components being deposited on a refractory three-dimensional structure and each specified amount being per litre of said refractory three-dimensional structure. Use according to claim 1 wherein (c) consists of palladium and rhodium. Use according to claim 1 or 2 wherein (d) is deposited in an amount of 0.2 to 1 g in total per litre of said refractory three-dimensional structure. Use according to any one of the preceding claims wherein said refractory inorganic oxide is at least one of activated alumina, silica, titania, zirconia, silica-alumina, alumina-zirconia, alumina-titania, silica-titania, silica-zirconia, titania-zirconia and zeolite. Use according to any one of the preceding claims wherein said refractory three-dimensional structure is a ceramic foam, an open flow ceramic honeycomb, a wall flow type honeycomb, a wall flow type honeycomb monolith, an open flow metal honeycomb, a metal foam or a metal mesh. Use according to any one of the preceding claims wherein (b) is deposited in an amount of 3 to 100 g in total per litre of said refractory three-dimensional structure. Use according to any one of the preceding claims wherein (a) is deposited in an amount of 10 to 200 g in total per litre of the said refractory three-dimensional structure. Use according to any one of the preceding claims wherein (c) is deposited in an amount of 0.5 to 5 g in total per litre of the said refractory three-dimensional structure. Use according to any one of the preceding claims wherein palladium and rhodium are deposited on said refractory three-dimensional structure in a deposit ratio of palladium/rhodium of 1//1 to 50/1.

Inventors: HORIUCHI MAKOTO, SAITO KOICHI, HORIUCHI, MAKOTO, SAITO, KOICHI
Applicants: NIPPON CATALYTIC CHEM IND, NIPPON SHOKUBAI KAGAKU KOGYO CO. LTD.
Application year: 1990
Main classification: B01D53
IPC codes: B01J23, B01D53, F02B3