CATALYST FOR SIMULTANEOUS DECOMPOSITION OF OXIDES OF NITROGEN AND OXIDATION OF CARBON MONOXIDE UNDER CYCLED OPERATING CONDITIONS

Abstract: This invention discloses an automotive exhaust gas treatment catalyst composed of one or more noble metals dispersed on CeO₂-RE₂O₃ carrier particles, where RE represents a rare earth metal. The ceria-rare earth oxide carrier for the noble metal facilitates the simultaneous oxidation of carbon monoxide, CO, and the decomposition of nitric oxide, NO. The catalyst is particularly useful when the air-fuel mixture to the automobile engine is continually cycled between fuel-rich and fuel-lean ratios compared to the stoichiometric air-fuel ratio.

Description: This invention relates to a catalyst for treating automobile exhaust gases to both oxidize carbon monoxide to carbon dioxide and to chemically reduce or decompose oxides of nitrogen to nitrogen. More specifically, this invention relates to an automotive exhaust gas treatment catalyst composed of one or more noble metals dispersed on ceria carrier particles as specified in the preamble of claim 1, for example, as disclosed in FR-A-2 584 388. Background of the Invention In order to reduce the amount of unburned hydrocarbons, carbon monoxide and oxides of nitrogen (generally designated as NO x , principally nitric oxide, NO and nitrogen peroxide, N₂O₄) in automotive exhaust gas to acceptable levels, special exhaust treatment catalysts and internal combustion engine operating practices have been developed. The successful conversion of the exhaust gas constituents is complicated by the requirement of simultaneously and rapidly oxidising carbon monoxide to innocuous carbon dioxide and reducing NO to nitrogen. In general, of course, the oxidation reaction is favoured by an abundance of oxygen, whereas the reduction reaction is inhibited by it. It has been found that, when the proportions of air and gasoline are balanced close to the stoichiometric ratio thereof, the simultaneous oxidation of CO and unburned hydrocarbons and the decomposition of NO can be accomplished to an acceptable degree. Catalysts called three-way catalysts have been devised for these operating conditions. In general, such catalysts comprise a dispersion of very small particles of platinum and rhodium on a carrier of high surface area, particulate alumina. These noble metals are scarce and expensive. They occur together in ores. Unfortunately, the present three-way catalysts require rhodium in an amount that exceeds its natural proportion in the noble metal-bearing ores. It is, therefore, desirable and an object of this invention to devise and provide a catalyst that requires a lower noble metal content in general, and less rhodium content in particular. It is, of course, necessary to also maintain or improve the efficiency of the automotive exhaust gas treatment. As stated above, it is a current practice to operate automotive engines at approximately the stoichiometric air-fuel ratio. Because of varying gasoline compositions and rapidly changing engine-operating parameters, it is not practical to operate the engine air-fuel ratio precisely at the stoichiometric proportion. Instead, it is the practice to continually and frequently cycle the air-fuel ratio of the mixture burned in the cylinders of the engine from just fuel-rich ratios compared to the stoichiometric ratio to just fuel-lean ratios compared to the stoichiometric ratio. Such cyclic variation of the air-fuel ratio is made possible by an oxygen sensor located in the exhaust manifold which detects whether the engine is then operating rich or lean of the stoichiometric ratio. The exhaust gas contains a very low amount of oxygen when the engine is running at a fuel-rich ratio and a much higher concentration of oxygen when the engine is then operating at a fuel-lean ratio. Continued signals from the oxygen sensor are processed by an engine computer control system to vary the fuel input to the engine so that the air-fuel ratio cycles between rich and lean virtually every second. Such rapid cycling of the air-fuel ratio produces an exhaust gas that over relatively short time increments averages stoichiometric air-fuel engine operation. The catalyst responds to such cycling of the air-fuel ratio. Therefore, it is desirable to have a catalyst which would permit more latitude in engine-operating conditions and less frequent cycling of the fuel control and supply system. It is an object of this invention to provide such a catalyst. Brief Summary of the Invention It has been found that the catalytic activity of noble metals for the simultaneous oxidation of carbon monoxide and decomposition of nitrogen oxide is improved under cyclic operating conditions if ceria (CeO₂) is substituted for alumina as the noble metal carrier. Furthermore, such catalyst activity is further improved to a remarkable degree when a chemically-reacted ceria-(rare earth metal)₂O₃ material is employed as the particulate carrier for the noble metal. Ceria-rare earth oxide solid solutions where ceria makes up 80 to 95 molar percent of the total are suitable for the practice of the present invention. Preferably, however, the rare earth oxide (RE₂O₃) constitutes about 10 to 15 molar percent of the ceria-based carrier. The catalyst carrier of the present invention is suitable prepared by mixing fine particles of ceria and of a rare earth metal (trivalent) oxide, preferably gadolinia (Gd₂O₃), and heating the mixture in air at about 1400°C to form a solid solution of the oxides. Such a chemically-reacted mixture of the tetravalent cerium oxide and trivalent gadolinium oxide produces a resultant crystal structure capable of temporarily absorbing additional oxygen. This carrier material is far superior to alumina (Al₂O₃) and significantly superior to ceria alone as a carrier for noble metals to catalytically promote the simultaneous oxidation of carbon monoxide and reduction of nitric oxide. The carrier of the invention would typically be employed in the form of pellets for pellet-type catalysts or in the form of a wash-coat for monolithic catalysts. It is perceived that the increased activity of mixed oxide catalyst carrier of the invention results from the ability thereof to temporarily store oxygen atoms that otherwise would be adsorbed on the active catalyst sites during further NO reduction. It is surmised from experimental work carried out on the invention that the activity of traditional alumina-supported noble metal catalysts for NO decomposition is decreased by the presence of oxygen adsorbed on the catalytic surfaces. Such adsorbed surface oxygen poisons or interferes with the activity of the catalyst during NO reduction that would otherwise be promoted during the fuel rich half of the engine cycle. The improved mixed oxide carrier of the invention better stores oxygen away from active catalytic sites of noble metals and thus promotes NO reduction. The mixed oxide carriers of the invention are converted into useful automotive exhaust treatment catalysts by dispersing one or more noble metals on the surface of the particles by suitable known techniques. For example, a solution of a compound of a noble metal such as rhodium or of a mixture of noble metals such as platinum and rhodium may be deposited onto the carrier particles. The carrier particles, of course, may be finely divided material or they may be in the form of pellets. After the particles have been soaked with the solution, they are calcined at a suitably elevated temperature to convert the noble metal compounds to finely dispersed particles of catalytic metal. Other objects and advantages of the present invention will be better understood and appreciated from a detailed description thereof which follows. Detailed Description Under current US Federal automotive exhaust emission standards, it is necessary to reduce the levels of each of the unburned hydrocarbons, carbon monoxide and nitrogen oxides in emitted exhaust gases to quite low levels. In order to meet these emission standards, automobile manufacturers have had to use rhodium in addition to platinum: rhodium is good for NO x reduction while platinum is good for CO and hydrocarbon oxidation. Rhodium is a noble metal found in ores with platinum and other metals but in appreciably lower quantities than platinum. Unfortunately, scarce rhodium has been used in automotive catalysts in greater proportions with respect to the accompanying platinum than the proportion of rhodium to platinum that occurs in mined ores. While it would be economically beneficial to reduce the amount of all noble metals required in automotive exhaust gases, a most immediate need is to reduce the requirements for the relatively scarce rhodium component. For this reason, the practice of the present invention is illustrated with examples that employ rhodium. As stated, rhodium is dispersed in the form of very small particles (along with particles of platinum) on a high surface area alumina in present commercially available automotive exhaust treatment catalysts. Whilst palladium and platinum are quite effective for the oxidation of unburned hydrocarbons and carbon monoxide, rhodium is presently much more effective for the reduction of nitrogen oxide. In the environment of automobile exhaust gases, it is realistic to view rhodium as catalyzing the reaction of the CO and NO to form CO₂ and nitrogen. In order to study and compare the effectiveness of rhodium on alumina and on two ceria-based carriers of the present invention, a series of rhodium catalysts was prepared as follows. A commercial grade of alumina was obtained having a specific surface (BET) area of 70 cm²/g. The alumina was in the form of a powder having a particle size in the range of 0.125 - 0.177 mm (80 to 120 mesh). Similarly, pure ceria (CeO₂) powder having a specific surface (BET) area of 0.3 cm²/g and 0.125 - 0.177 mm (80 to 120 mesh) size was obtained. A special binary solid solution of ceria with gadolinia was also made as follows. This material, which can be referred to as "activated ceria", also had a specific surface (BET) area of 0.3 cm²/g and a 0.125 - 0.177 mm (80 to 120 mesh) particle size. Ceria powder less than 0.044 mm in size (-325 mesh) and gadolinia (Gd₂O₃) powder less than 0.044 mm in size (-325 mesh) were thoroughly mixed in molar proportions of 89 parts ceria to 11 parts gadolinia. The powdery mixture was heated at 1400°C in air for 16 hours and then slowly cooled to room temperature in ambient air. The resulting binary solid solution of oxides was crushed, ground and screened to obtain a 0.125 - 0.177 mm (80 to 120 mesh) particle size fraction. The proportions of gadolinia and ceria were such that the binary mixture crystallized in the fluorite-type cubic lattice of ceria. However, it was perceived that the replacement of some of the ceria with gadolinia would introduce defects in this cubical crystal structure that could accommodate additional oxygen. Catalysts were prepared using the three types of carriers as described below. In general, the practice described in US-A- 4,380,510 was followed so as to deposit the noble metal at the outer surface of each of the carriers. A dilute solution (0.075 weight percent) of a rhodium salt [(n-C₄H₉)₄N]₂ [Rh(CO)Br₄]₂ in acetone was prepared. Measured portions of the solution were added to equal volumes of each of the alumina powder, ceria powder and activated ceria powder (CeO₂-Gd₂O₃) such that each powder received the same amount of rhodium. The rhodium salt was added to the alumina in the amount of 0.09 weight percent rhodium. The ceria powder and activated ceria powder had substantially the same density, which was higher than the density of the alumina powder. Accordingly, the rhodium coating on each of these ceria powders was only 0.04 weight percent in order to compensate for the difference in density. The acetone-impregnated oxides were dried overnight in air at room temperature, heated slowly in flowing air up to 500°C, and then calcined in air at 500°C for four hours. The rhodium dispersion on each carrier material was confirmed by CO chemisorption assuming 1:1 stoichiometry between a surface rhodium atom and an adsorbed CO molecule. The properties of the three catalysts are summarized in the following table (Table 1). CATALYST PROPERTIES Catalysts Rh Loading (wt%) Total Catalyst Weight (g) BET Area (cm²/g) Particle Size mm (mesh) Rh/Al₂O₃ 0.09 0.22 70 0.125 - 0.177 (80-120) Rh/CeO₂ 0.04 0.49 0.3 0.125 - 0.177 (80-120) Rh/CeO₂-Gd₂O₃ 0.04 0.49 0.3 0.125 - 0.177 (80-120) A catalytic reactor was prepared using a 3.2 mm outer diameter stainless steel tube. The depth of the catalyst bed in the tube was 1 cm. The tube was placed in a 30 cm long tubular furnace. Before each experimental run with a catalyst, the reactor containing the catalyst particles was flushed with 1 volume percent carbon monoxide in specially treated oxygen-free helium at 600°C for half an hour to reduce the catalyst metal. The reactor temperature was then reduced to the desired operating temperature of 500°C and held at that temperature for one hour with the flow of helium carrier gas containing approximately one part per million of CO. This ensured substantially complete desorption of CO as well as elimination of oxygen contamination on the surface of each catalyst before it was tested. The apparatus was adapted to alternately admit CO and NO into the helium carrier gas, each in the amount of about 800 parts per million based on the helium. The total gas flow rate was 50 cm³/minute at 101 kPa. The rhodium/alumina catalyst was tested first in the reactor following the above-described catalyst preparation procedure. Cyclic experiments of CO oxidation and NO reduction were carried out at 500°C by alternately feeding CO and NO to the reactor in the helium carrier gas in the proportions and flow rates described above. In the first experiment, a CO pulse of 10 seconds duration was followed by an NO pulse of 10 seconds duration, which was followed by another CO pulse, and so on. The effluent from the rhodium-alumina reactor outlet was monitored by a mass spectrometer at the reactor outlet. A study of the recorder output of the mass spectrometer revealed that during each 10 second CO half-cycle, the Rh/Al₂O₃ catalyst was active for CO oxidation for about 5 seconds and then the activity disappeared. On the other hand, during each NO half-cycle, the catalyst was active for NO reduction for about 5 seconds and then the reduction activity ceased. These results were observed through repeated NO and CO half cycles. Integration of the mass spectrometer output during testing of the Rh/Al₂O₃ catalyst showed that the conversion of CO and NO was 76.5 percent. Note that in this experiment the only oxidizing material present was the NO and the only reducing material present was the CO, and so the conversion of one of these materials resulted in the conversion of the other. In fact, the mass spectrometer confirmed the appearance of carbon dioxide and nitrogen in the exhaust gas in exact correlation with the disappearance of CO and NO. A similar experiment was then carried out using the Rh/CeO₂ catalyst. The reactor was sealed and the catalyst was pretreated as described above before the alternating 10 second half-cycle pulses of CO and NO in helium were flowed through the furnace and catalyst bed at 500°C. Integration of the mass spectrometer data with this catalyst showed an 86.5 percent conversion of CO and NO. Thus, the substitution of the ceria carrier for the alumina as a support for the rhodium catalyst produced a substantially higher conversion of CO and NO than the alumina support despite relatively long duration pulses of the oxidizing or reducing medium to the catalyst bed. The CO half-cycle is viewed as being sufficiently similar to a fuel-rich engine-operating mode and the NO half-cycle as being sufficiently similar to a fuel-lean engine-operating cycle as regards catalyst performance to conclude that the ceria carrier could permit reduction in rhodium content of the catalyst and/or less frequent cycling of the air-fuel supply to the engine. After evaluating the rhodium-alumina and rhodium ceria catalysts, the same experiment was conducted utilizing the rhodium-ceria-gadolinia catalyst. The CO and NO were pulsed in 10 second half-cycles as described above at 500°C after suitable pretreatment of the catalyst. In this experiment, mass spectrometer data indicated that 98.2 percent of the CO and NO had been converted to carbon dioxide and nitrogen, respectively. This is viewed as being a very dramatic improvement over the ceria carrier and certainly a very large improvement over the presently used alumina carrier catalyst. Fresh quantities of the same three rhodium catalysts were then sequentially tested in the reactor using repeated 30 second pulses of the CO and NO. As expected, the conversions of CO and NO were reduced during these lengthened cycles. A reduction in the conversion was expected because it was assumed that oxygen produced by NO reduction would exceed the storage capacity of the respective carriers and accumulate on the noble metal catalyst sides, thus impeding the desired conversion reactions. Indeed, during the 30 second pulse half-cycles of CO and NO, the Rh/Al₂O₃ catalyst provided a conversion of only 50.8 percent of the CO and NO. The Rh/CeO₂ catalyst provided a 57.8 percent conversion. However, the Rh/CeO₂-Gd₂O₃ catalyst still provided an 82.5 percent conversion of CO and NO. It is to be noted that despite a three-fold lengthening of the CO and NO exposure to the catalyst, the ceria-gadolinia carrier catalyst of the invention still provided a larger, conversion of the CO and NO than did the alumina carrier catalyst during the 10 second half-cycle pulses of CO and NO. It is apparent that the ceria-trivalent rare earth metal oxide carrier for noble metal catalyst disclosed in this application has the capability of successfully converting CO and NO during long cycle variations in engine air-fuel ratio. The carriers are formed by the reaction of ceria with the oxide of a trivalent rare earth metal such as gadolinium, erbium, neodymium, praseodymium, samarium and lanthanum. By combining the tetravalent oxide of cerium with the oxide of a different and trivalent rare earth metal, it is believed that the reacted solid solution forms a crystalline material capable of sufficient oxygen storage to accommodate longer exposure of noble metal sites to oxygen. As previously stated, preferably the carrier is formed of 85 to 90 molar percent CeO₂ and 10 to 15 mole percent of the oxides of one or more trivalent rare earth metals. Suitably, however, carriers may be prepared containing about 5 to 20 mole percent of the trivalent rare earth metal oxides. Noble metals such as, e.g., platinum and/or palladium and/or rhodium may be dispersed on the carrier by any procedure that deposits the materials in the form of very fine particles. Usually the noble metals are deposited by preparing solutions of their compounds and wetting the carrier with such solutions. Aqueous or nonaqueous solutions may be used. The metals may be deposited sequentially or from the same solution. The ceria-rare earth oxide combination carrier of the invention is particularly effective for promoting the simultaneous oxidation of CO and decomposition of NO.

Claims: A catalyst for treating automotive exhaust gases containing both carbon monoxide (CO) and nitric oxide (NO), the catalyst comprising one or more noble metals finely dispersed on carrier particles containing ceria and a rare earth metal oxide, RE₂O₃, where RE represents a rare earth metal, characterised in that the carrier particles comprise a crystalline, solid solution of 80 to 95 mole percent ceria (CeO₂) and 5 to 20 mole percent of said rare earth metal oxide, which crystalline, solid solution is capable of temporarily absorbing additional oxygen. A catalyst according to claim 1, in which the noble metal is platinum, rhodium, or a mixture of platinum and rhodium. A catalyst according to claim 1 or 2, in which the carrier particles comprise a crystalline, solid solution of 85 to 90 mole percent CeO₂ and 10 to 15 mole percent RE₂O₃. A catalyst according to claim 1, 2 or 3, in which the rare earth metal is gadolinium. A method of treating the exhaust gas produced from the operation of a hydrocarbon-fuelled internal combustion engine comprising continually cycling an air-fuel mixture supplied to the engine between a fuel-rich condition and a fuel-lean condition whilst contacting the exhaust gas with a catalyst comprising one or more noble metals deposited on ceramic carrier particles, characterised in that the carrier particles comprise particles of a crystalline, solid solution formed from heating a mixture comprising 80 to 95 mole percent CeO₂ and 5 to 20 mole percent RE₂O₃, where RE is a rare earth metal, in air at 1400°C. A method of treating the exhaust gas produced from the operation of a hydrocarbon-fuelled internal combustion engine according to claim 5, in which the crystalline, solid solution is formed from a mixture comprising 85 to 90 mole percent CeO₂ and 10 to 15 mole percent RE₂O₃. A method of treating the exhaust gas produced from the operation of a hydrocarbon-fuelled internal combustion engine according to claim 5 or 6, in which the rare earth metal RE is gadolinium.

Inventors: CHO, BYONG KWON, STOCK, CHRISTOPHER JOSEPH
Applicants: GENERAL MOTORS CORPORATION
Application year: 1988
Main classification: B01D53
IPC codes: B01D53, B01J23