Improved alumina-ceria catalyst washcoat

Abstract: A high surface area ceria-alumina washcoat is disclosed in which the ceria is prepared from cerium acetate.

Description: This invention pertains to automotive exhaust catalysts. More particularly, it relates to more durable alumina/ceria carriers or washcoats for noble metals in catalytic converters for gasoline-fuelled automotive engines. Thermally stable, high surface-area (e.g., > 100 m²/g) alumina in the form of small beads or as micrometre-sized washcoat particles has long been used as a carrier for finely-dispersed particles of noble metals in the treatment of automobile exhaust gases. More recently, ceric oxide (CeO₂, also called ceria) has been incorporated into the alumina particles or mixed with them as the carrier for the dispersed particles of platinum and/or palladium and rhodium. Initially, ceria was used in small amounts, about 2 to 3 percent by weight of the alumina. Now it is being used in a larger proportion relative to the alumina, about 20 to 40 percent by weight of the carrier. EP-A-314,058 discloses a catalyst comprising an alumina-ceria support which is prepared by forming a slurry of alumina and a cerium salt, and coating this slurry onto a support body, followed by calcination. It is now necessary to give attention to how to best utilize the ceria from the standpoint of catalyst cost and performance. EP-A-321,949 discloses a washcoat formed from a slurry of alumina and lanthanum oxide. Accordingly, it is an object of the present invention to provide an improved method of preparing an alumina-ceria catalyst support for the treatment of automotive engine exhaust gases. To this end, a method in accordance with the present invention is characterised over EP-A-314,058 by the combination of features in Claim 1. It is a more specific object of the present invention to provide a method of forming a high surface-area ceria for combination with alumina as a catalyst carrier washcoat on a monolithic-type catalyst substrate. The resultant washcoat improved catalyst durability as reflected in the high conversion of HC, CO and NOx even after prolonged thermal aging of the catalyst. It is another object of the invention to provide a more durable alumina-ceria catalyst carrier washcoat for ceramic or metallic monolith-type catalyst substrates. In accordance with a preferred embodiment of the present invention, these and other objects and advantages are accomplished as follows. A new method is used for preparing a thermally-stable, high surface-area mixture of alumina and ceria particles that make up a washcoat-type carrier for noble metal catalysts for the conversion of automotive exhaust gas constituents. A specific precursor material is used for the preparation of the ceria in that cerium acetate is used as the starting material. When cerium acetate is calcined, a ceric oxide is formed that is chemically the same, of course, as other cerias, but its unique particulate form provides it with characteristics particularly suitable in the catalytic converter environment. In general, one starts with a wet cerium carbonate which is treated with acetic acid to form a cerium acetate. A major portion of the cerium acetate is dried and calcined suitably at a temperature of about 425°C to 450°C to form a high surface-area ceric oxide, CeO₂ (HSA ceric oxide). A mixture of 70 mole percent high surface-area cerium oxide and 30 mole percent of the cerium acetate is then prepared. This mixture is wet-milled with alumina in proportions that will ultimately yield about 50 to 80 weight percent alumina and the balance cerium oxide. Small amounts of other materials such as known oxide stabilizers may be added if desired. The wet mixture is milled for a period of time so as to achieve a substantially uniform mixture of the ceric oxide and alumina and cerium acetate. Some of the cerium acetate dissolves in the water and the solution is adsorbed onto the alumina. Ultimately, a mixture of average particle size of about 0.5 to 1 micrometre is obtained. The viscosity of the milled slurry may be adjusted if necessary, and the material is then applied as a thin layer to the exhaust gas treatment surfaces of a ceramic or metal monolith configured for the treating of automotive engine exhaust gases. The coated monolith is then suitably dried and heated in air (calcined) at a temperature of 425°C or so to obtain a film-like coating of fine ceria particles and alumina particles. The alumina particles may also carry some ceric oxide. This washcoated monolith body may then be treated by the application thereto of finely-dispersed noble metal particles. This is accomplished by providing a suitable solution of noble metal salts and impregnating the washcoat layer of the monolith with the solution to form a suitable loading of one or more noble metals such as, for example, platinum and rhodium. After impregnation with the noble metal, the washcoated monolith is again dried and calcined to fix the finely-dispersed noble metal particles on the washcoat layer. Alternatively, the precious metal precursors can be added to the mill and mixed with the milled slurry. The monoliths are then coated with the activated slurry. It has been found that a washcoat layer thus prepared displays and retains excellent conversion efficiency for each of the unburned hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) present in exhaust gases, even after prolonged exposure of the catalyst to high-temperature exhaust gas. Whilst the ceria and alumina each contribute to the performance of the washcoat in providing the durability of the catalytic material, it is believed that the use of cerium acetate as a precursor for the formation of the ceria provides a significant improvement in the long-term performance of the catalyst. These and other objects and advantages of the invention will be more clearly understood in view of a detailed description which follows. A high surface-area cerium oxide is an important component of a catalyst composition prepared by the method of the present invention, and a preferred method of preparing such a cerium oxide compound is employed so that it is relatively thermally-stable and retains usable surface area despite prolonged exposure to oxygen and other exhaust constituents at elevated temperatures. It is preferable to prepare the high surface-area cerium oxide from a wet cerium carbonate (containing about 35 percent by weight water) by first heating the carbonate with acetic acid and calcining the product (presumably cerium acetate) in air to obtain a ceria with a BET surface-area of approximately 80 to 100 metres²/gram. The high surface-area cerium oxide can be prepared from other cerium acetate starting material, if desired. Example of the Preparation of High Surface-Area Cerium Oxide Three thousand grams of wet, solid Ce₂(CO₃)₃ (containing about 35 percent water) was reacted with 1569 grams of glacial acetic acid. Carbon dioxide was evolved. The resulting product (cerium acetate) was dried and calcined at about 425°C for about two hours in a stream of air flowing at a rate of about 0.85 metre³/minute (30 standard cubic feet per minute (SCFM)). A thermally-stable, high surface-area (HSA) ceric oxide was produced. Ceric oxides prepared by this method typically have BET surface areas in the range of 80 to 100 m²/g. Preferably, a ceric oxide with a surface-area in excess of 80 m²/g is obtained. The inventors were interested in determining how the surface area of such ceric oxide was affected by heating in air at 950°C for four hours. The inventors view such a heating schedule as useful in assessing the performance of automotive exhaust treatment catalytic materials in resisting thermal degradation. It was found that, after exposure at 950°C in air for four hours, the calcined, acetate-based ceric oxide retained a BET surface area of 12 to 16 m²/g. Although considerable surface area is lost by such heating, such residual area is very useful in providing durability in a catalyst produced from the method of the invention. It is recognized that other forms of ceric oxide can be prepared by the calcination of other cerium compounds. However, these other forms have not retained as much of their surface-area in thermal aging tests. Representative ceric oxides have been prepared by the following steps: (a) cerium carbonate was calcined in air at 425°C to CeO₂; (b) wet cerium nitrate was calcined to CeO₂ at 425°C; and (c) ceric nitrate was converted to ceric citrate and calcined to ceric oxide at 425°C. The cerium carbonate-based ceric oxide had an initial BET surface area of 60 to 70 m²/g, but upon heating at 950°C for four hours, its effective surface area fell to about 1.5 m²/g. The cerium nitrate-based ceric oxide had an initial BET surface area of 50 to 60 m²/g and its surface area diminished to 6 m²/g after the heating schedule at 950°C. The ceric citrate-based ceric oxide had an initial surface area of 60 to 70 m²/g and a post-950°C heating regimen surface area of about 6 m²/g. Example of the Preparation of the Washcoat of this Invention A liquid slurry was prepared containing 428 grams of gamma alumina, 22 grams of alumina monohydrate, 122 grams of HSA cerium oxide (prepared from cerium acetate as described above), 97 grams of cerium acetate (prepared from cerium carbonate as described above), 10 grams of acetic acid and 571 grams of water. This liquid slurry mixture was prepared so that it had an oxide content of about 50 percent by weight. The mixture may suitably contain 45 to 55 percent by weight of oxides. The content of the above mixture was such that it would yield 28 percent by weight of ceric oxide and 72 percent by weight of alumina. The cerium-containing constituents of the mixture are proportioned such that 70 percent of the cerium present is provided by the HSA cerium oxide and 30 percent of the cerium present by the cerium acetate. About 5 percent of the alumina is added as alumina monohydrate. The acetic acid is employed to peptize the alumina monohydrate, which will provide bonding strength after washcoating. Optionally, other oxides such as nickel oxide, lanthanum oxide or potassium oxide can be added to the formulation for their known effect, for hydrogen sulphide suppression and/or improvement in the conversion performance of the catalyst. The proportion of HSA cerium oxide to cerium acetate illustrated above (i.e., 70 mole percent to 30 mole percent) is preferred. However, HSA cerium oxide may make up 50 to 80 mole percent of the oxide acetate mixture and cerium acetate 20 to 50 mole percent of the mixture. Suitably, 3 to 8 weight percent of the alumina is added as aluminum hydrate. The above-specified 1250 gram slurry was milled in a container with 4000 grams of 6.35mm (quarter inch) alumina mill balls at room temperature for three hours. The viscosity of the recovered slurry from the mill was adjusted to about 400 centipoise (suitably 200 cp to 600 cp). The viscosity of the slurry can be adjusted if necessary by adding water to decrease viscosity or small amounts of acid to increase viscosity. The particle size in the resulting slurry is about 0.5 to 1 micrometres. The slurry is now ready to be applied as a washcoat layer on the exhaust gas-contacting surfaces of a suitable monolith-type catalyst support device. For purposes of illustration, a 1393 cm³ (85 cubic inch) ceramic monolith (cordierite) is selected with 62 cells (longitudinal exhaust flow passages) per square centimetre (400 cells per square inch). About 1000 grams of the slurry were drawn down through the cell openings to coat the walls of the cells with the washcoat material. After the excess slurry was removed, the coated monolith was dried for eight hours at 120°C and calcined at 427°C in air for one hour. The weight of the calcined washcoat was 275 grams. At this stage, the cordierite monolith carried a thin coating of the alumina-ceria washcoat material. The washcoat is characterized as a mixture of high surface-area ceria particles and high surface-area gamma alumina particles that are about 0.5 to 1 micrometres in particle size. It is believed that the alumina particles also carry some ceric oxide due to impregnation thereof by water-dissolved cerium acetate. Following the formation of the calcined washcoat layer, platinum and rhodium were then loaded onto the washcoat. Aqueous solutions of amine chloride complex compounds of platinum and rhodium were formed, and the solutions were impregnated into the alumina and ceria particles of the washcoat. The solution contained suitable quantities of platinum and rhodium such that the monolith carried a total of 1.182 grams (0.038 troy ounces) of platinum metal and 0.1182 grams (0.0038 troy ounces) of rhodium metal on the walls of its cells after calcination at 427°C for one hour in air. A number of like monolithic catalyst bodies have been prepared and assembled ("canned") in stainless steel catalytic converters of current commercial manufacture for testing. Testing and Comparison with Other Washcoats Before testing the above prepared catalyst for its exhaust gas constituent conversion efficiency, it was subjected to an accelerated high-temperature engine-operating schedule for 75 hours. The aging of the converter was done in conjunction with a Chevrolet 4.3 litre engine operated in conjunction with a standard water brake dynamometer. The engine speed and dynamometer loading were conducted in accordance with a rapid aging test (RAT) for the catalyst (catalyst inlet temperature of 700°C plus power enrichment and air injection to produce 900°C temperature spikes) for 75 hours. The aging effect on the catalyst of such a 75 hour accelerated schedule is known to be equivalent to that of a 50,000 mile normal driving schedule. Following the RAT catalyst aging test, the converter was then operated in connection with a 2.5 litre engine on a Buick Somerset vehicle in accordance with the U.S. Federal Test Procedure (FTP test). The alumina/ceria washcoat carrier in combination with the described three-way platinum-rhodium catalyst obtained or produced conversion efficiencies as follows: HC, 86 percent; CO, 88 percent and NOx, 81 percent. Such conversion efficiencies on the FTP test cycle following the rapid aging test are considered to be very good. For purposes of comparison, alumina-ceria washcoats based on other ceria precursor materials were prepared for evaluation. Each of the washcoats was prepared to contain gamma alumina and a ceria in the proportions of 72 weight percent alumina and 28 weight percent ceria. The washcoats were calcined as described above and impregnated with the same loadings of platinum and rhodium applied in the same manner as described above onto a cordierite ceramic monolith of the same size. The thermal aging and testing of the different washcoat catalysts was precisely as described above in connection with the novel washcoat material prepared according to the invention. In a first comparison washcoat, the precursor of the ceria was cerium carbonate. All of the ceria content of the washcoat came from cerium carbonate Ce₂(CO₃)₃ which was added into the mill and calcined as a part of the washcoat. The FTP cycle conversion efficiencies for the catalyst based on the cerium carbonate precursor material were respectively: HC, 80 percent; CO, 82 percent and NOx, 72 percent. A second washcoat comparison sample was prepared in which the sole source of the ceria was cerium nitrate, Ce(NO₃)₃. An aqueous cerium nitrate solution was added with the alumina in the ball mill, and upon coating of the cerium nitrate-impregnated alumina on the cordierite monolith and calcining, the cerium nitrate was converted to ceric oxide. Following impregnation with platinum and rhodium and aging in accordance with the RAT schedule, this catalyst demonstrated the following conversion efficiencies on the FTP procedure: HC, 83 percent; CO, 83 percent and NOx, 79 percent. A third comparison washcoat material was prepared utilizing high surface-area ceric oxide as the sole source of ceria in conjunction with the gamma alumina. The HSA ceria and gamma alumina were ball-milled together and the resulting slurry applied to a 1393 cm³ (85 cubic inch) volume cordierite monolith. After calcining, a washcoat of finely divided particulate ceria and gamma alumina was obtained. The washcoat was impregnated as with the above samples with platinum and rhodium and the catalyst subjected to the rapid aging test. Following the rapid aging test, this catalyst demonstrated the following conversion efficiencies during the FTP test procedure: HC, 82 percent; CO, 84 percent and NOx, 74 percent. A comparison of the conversion efficiencies of the four different washcoat materials reveals that the alumina-ceria washcoat prepared by the practice of the present invention yielded higher conversion efficiencies for each of the unburned hydrocarbons, carbon monoxide and nitrogen oxides in the exhaust gas tested than any of the other catalysts produced for any of the exhaust gas constituents. Measured as a percent reduction in the amount of unconverted gases, the washcoat formulation prepared according to the present invention provides a 17 percent better reduction in the unconverted hydrocarbons than the best other performing catalyst with respect to hydrocarbons. The converter using this washcoat formulation produced a 25 percent better reduction in the unconverted carbon monoxide than the next better converter and a 9 percent better reduction in the unreacted NO when compared to the next best washcoat performance. The inventors believe that the use of the combination of high surface-area ceric oxide and cerium acetate with the high surface-area alumina in the preparation of the washcoat according to the present invention enhances the exhaust gas treatment process by more than one mechanism. The distinct particles of HSA ceric oxide contribute to the conversion process. However, it is also likely that some of the dissolved ceric acetate used in the practice of the process of the invention permeates the structure of the alumina particles and introduces ceric oxide into the alumina for high temperature transformation control and high surface-area stabilization. Thus, it appears that the washcoat benefits from the presence of distinct particles of the specially-prepared HSA CeO₂ and from the fact that the cerium acetate impregnated into the alumina particles may also be converted to HSA CeO₂ upon calcination and thus constitutes a portion of the alumina particles. In any event, whether this theory of the inventors is correct or not, it has been demonstrated that, by using a combination of the special HSA CeO₂ and cerium acetate in the preparation of alumina-ceria washcoats, benefit is obtained in high conversion efficiencies of converters using such washcoats, especially after aging of the catalyst. Further comparison tests of alumina-ceria washcoat materials have been run in which the ceria was applied to the alumina by a post-impregnation practice following calcination of the alumina in a washcoated film on a cordierite monolith substrate. In other words, a washcoat of alumina particles was first formed on the cordierite monolith and then the calcined alumina was subsequently impregnated with an aqueous solution of a ceric oxide precursor material. In one example, a washcoat was formed containing 85 percent alumina and 15 percent by weight ceria by impregnating the calcined alumina with a cerium nitrate solution. The cerium nitrate-impregnated alumina was then calcined for one hour in air at 427°C to form the alumina-ceria washcoat. This washcoat in turn was impregnated with a platinum-rhodium catalyst in the same loadings as described above. The platinum-rhodium impregnated cordierite monolith was then placed in a stainless steel converter can and thermally aged as described above and then tested in accordance with the U.S. Federal Test Procedure. The conversion efficiencies for this cerium nitrate-impregnated alumina were as follows: HC, 67 percent; CO, 65 percent and NOx, 59 percent. It is seen that these conversion efficiencies are not comparable to those obtained by the catalyst prepared in accordance with the practice of the present invention. Another comparison catalyst was prepared in which a calcined alumina-washcoated cordierite monolith was impregnated with cerium nitrate to form a ceria content of only 8 percent. After completion of the catalyst-making process, thermal aging and testing by the FTP test, the conversion efficiencies were found to be as follows: HC, 68 percent; CO, 62 percent and NOx, 59 percent. Finally, a catalyst was prepared in which a calcined alumina washcoat on a cordierite monolith was impregnated with a cerium acetate solution of sufficient volume to provide an alumina-ceria washcoat containing 8 percent ceria. After completion of the catalyst-making procedure, the RAT aging practice and finally testing by the FTP procedure, exhaust gas conversion efficiencies were obtained as follows: HC, 80 percent; CO, 75 percent and NOx, 70 percent. It is seen that utilizing a cerium acetate to post-impregnate an alumina washcoat provides a better washcoat material than impregnation with cerium nitrate. However, the final catalyst performance after aging is still not as good as when both a cerium acetate and the high surface-area ceria are used in the preparation of the alumina-ceria washcoat. Thus, the inventors conclude that they have discovered an improved process for making alumina-ceria washcoats in which both a high surface-area ceric oxide prepared from cerium acetate and cerium acetate are employed in the preparation of the alumina-ceria washcoat material. In summary, the method aspect of the present invention is practiced as follows. The use of high surface-area aluminas are preferred, such as, for example, the transitional chi, gamma, delta and theta aluminas. These are thermally-stable aluminas which retain a preponderance of their high surface area despite exposure to hot exhaust gases. The final washcoat preferably contains 50 to about 80 percent by weight alumina and correspondingly 20 to 50 percent ceric oxide prepared in accordance with the practice of the present invention. Other known oxide stabilizers or modifiers may be used, but the proportions of the ceria and alumina should be in the stated ranges. A high surface-area ceric oxide is used which is prepared from cerium acetate as described above. In the preparation of the washcoat coating mixture, cerium acetate is also included with the high surface-area alumina oxide and alumina. It is preferable to use a mixture of 70 mole percent high surface area cerium oxide to 30 mole percent cerium acetate. However, other mixtures can be used. Mixtures containing equal molar parts of cerium acetate and HSA cerium oxide can be used up to mixtures containing only 20 percent cerium acetate and 80 percent HSA cerium oxide. From the viewpoint of a product, the washcoat can be viewed as containing HSA ceric oxide, which is based on a cerium acetate precursor material. Furthermore, the final product is the result of the use of such a high surface-area ceric oxide in conjunction with additional cerium acetate in the preparation of the final alumina-ceria washcoat mixture. Whilst the practice of the present invention has been described in conjunction with a ceramic monolith, it is to be understood that the washcoat may also be used on metallic-type monolith catalyst carriers. The use of the washcoat in conjunction with a three-way platinum-rhodium catalyst has also been described. It is to be understood that the washcoat is also useful in conjunction with other noble metal mixtures such as palladium or mixtures of palladium with platinum or rhodium. It is also to be understood that suitable noble metal compounds may be wet-milled with the oxide mixture prior to washcoat application, or the noble metal compounds may be applied by a post-impregnation process to the calcined washcoat as described. While the present invention has been described in terms of a few specific examples thereof, it will be appreciated that the scope of the present invention is to be considered limited only by the following claims.

Claims: A method of making a thermally-stable, high surface-area, particulate alumina-ceria catalyst support for use in a catalytic converter for the treatment of the exhaust from a gasoline-fuelled automotive vehicle engine, the method comprising calcining cerium acetate thereby forming a cerium oxide having a surface area of at least 80 m²/g, wet-milling a mixture comprising said cerium oxide, cerium acetate and a high surface-area alumina in proportions such that the overall cerium content as cerium oxide and aluminium content as aluminium oxide are respectively 20 to 50 percent by weight and 80 to 50 percent by weight thereby obtaining a uniform slurry mixture of fine particles, applying the slurry mixture as a washcoat layer to exhaust gas flow-contacting surfaces of a catalyst support body for said catalytic converter, and drying and calcining the coated body thereby converting the wet washcoat layer to an adherent oxide coating in which the cerium oxide and alumina are in said proportions, the oxide coating being suitable for carrying dispersed particles of one or more noble metals thereon. A method of making a thermally-stable, high surface-area, particulate catalyst support according to claim 1, in which the cerium acetate is prepared by the reaction of a wet mass of cerium carbonate with acetic acid. A method of making a thermally-stable, high surface-area, particulate catalyst support according to claim 1, in which the adherent oxide coating is impregnated with a solution of noble metal salt and is subsequently dried and calcined thereby forming a dispersion of the noble metal thereon. A method of making a thermally-stable, high surface-area, particulate catalyst support according to claim 1, in which a noble metal catalyst precursor compound is included in the wet-milling mixture. A method of making a thermally-stable, high surface-area, particulate catalyst support according to claim 1, in which the cerium oxide constitutes 50 to 80 mole percent of the total of the content of cerium oxide plus cerium acetate present in the wet-milling mixture. An alumina-ceria catalyst washcoat support on a monolithic-type support body wherein the washcoat support is prepared by the method of any one of claims 1 to 5, including calcining cerium acetate thereby forming a cerium oxide having a surface area of 80 m²/g, and wet-milling a mixture comprising said cerium oxide, cerium acetate and a high surface-area alumina.

Inventors: BEDFORD RAYMOND EARL, TSANG CHIH-HAO MARK, BEDFORD, RAYMOND EARL, TSANG, CHIH-HAO MARK
Applicants: GEN MOTORS CORP, GENERAL MOTORS CORPORATION
Application year: 1991
Main classification: B01J37
IPC codes: B01D53, B01J23, B01J21, B01J32, B01J37