Process for producing exhaust-gas-purifying catalyst

Abstract: A process for producing an exhaust-gas-purifying catalyst includes the steps of immersing a porous support into a solution including a noble metal catalyst ingredient, thereby preparing a support with a noble metal catalyst ingredient loaded thereon, and thermally treating the noble-metal-catalyst-ingredient-loaded support at a temperature of 800 °C or more in non-oxidizing atmosphere. In the thermally-treating step, the noble metal catalyst ingredient is prevented from being oxidized to its oxide, and accordingly it is inhibited from migrating in vapor phase. Further, in the thermally-treating step, pores of the porous support are contracted so that they surround the noble metal catalyst ingredient closely therein. All in all, the noble metal catalyst ingredient is prevented from sintering. Thus, the resulting exhaust-gas-purifying catalyst is of superb heat-resistance, and it can be applied to elevated temperatures of 800 °C or more in fuel-lean atmosphere.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a process for producing an exhaust-gas-purifying catalyst which is capable of simultaneously purifying three components, such as hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NO x ), included in exhaust gases emitted from internal combustion engines for automobiles etc. An exhaust-gas-purifying catalyst produced in accordance with the present invention is especially useful for applications in fuel-lean atmosphere in which oxygen is included in an amount of the stoichiometric point, or more than required for oxidizing carbon monoxide (CO) and hydrocarbons (HC) in exhaust gases. Description of Related Art As catalysts for purifying automotive exhaust gases, there have been employed 3-way catalysts so far which oxidize CO and HC and reduce NO x simultaneously to purify exhaust gases. For example, the 3-way catalysts have been known widely which comprise a heat resistant support formed of cordierite, a porous catalyst carrier layer formed of gamma-alumina and disposed on the support, and a noble metal catalyst ingredient selected from the group consisting of Pt, Pd and Rh and loaded on the porous catalyst carrier layer. Recently, it has been likely to install an exhaust-gas-purifying catalyst directly below a manifold which is disposed adjacent to an engine, and accordingly an exhaust-gas temperature has been likely to increase during high-speed traveling. As a result, an exhaust-gas-purifying catalyst has been more likely to be exposed to elevated temperatures. However, in conventional exhaust-gas-purifying catalysts, thermal degradation might be developed in the gamma-alumina by high-temperature exhaust gases. The sintering of the noble catalyst ingredient resulting from the thermally-degraded gamma-alumina might possibly deteriorate the catalytic capability of conventional exhaust-gas-purifying catalysts. In order to solve the possible problems associated with conventional exhaust-gas-purifying catalyst, Japanese Unexamined Patent Publication (KOKAI) No. 4-122,441 discloses a process for producing an exhaust-gas-purifying catalyst. In this prior production process, alumina is utilized which has been thermally treated in advance, and a noble metal catalyst ingredient is loaded on the thermally-treated alumina. Since the alumina has been thermally treated in advance by the production process, the thermal degradation is hardly developed in the resulting exhaust-gas-purifying catalyst even when it is exposed to elevated temperatures, and accordingly the noble metal catalyst ingredient is little sintered. Thus, the resulting exhaust-gas-purifying catalyst can stably exhibit its catalytic capability. When driving automobiles, especially when driving automobiles in urban areas, the automobiles are accelerated and decelerated frequently. Consequently, the air-fuel ratio varies frequently in the range of from the values adjacent to the stoichiometric point (or the theoretical air-fuel ratio: 14.6) to the fuel-rich side. In order to satisfy the low fuel consumption requirement during the driving conditions such as in the above-described urban areas, it is necessary to operate the automobiles on the fuel-lean side where the air-fuel mixture containing oxygen as excessive as possible is supplied to the engines. However, it has been found that the exhaust-gas-purifying catalyst produced by the process disclosed in the aforementioned patent publication considerably suffers from the sintering of noble metal catalyst ingredient when it is operated under the fuel-lean conditions and when it is subjected to elevated temperatures of 800 °C or more. Thus, the exhaust-gas-purifying catalyst is degraded in terms of catalytic capability. US Patent No. 6 017 846 is directed to exhaust gas catalysts which may be obtained by applying platinum and rhodium to a ceria - alumina support. SUMMARY OF THE INVENTION The present invention has been developed in view of the aforementioned circumstances. It is therefore an object of the present invention to provide a process for producing an exhaust-gas-purifying catalyst whose noble metal catalyst ingredient is securely inhibited from sintering when the exhaust-gas-purifying catalyst is operated under the fuel-lean conditions and when it is subjected to elevated temperatures of 800 °C or more. According to the present invention carrying out the aforementioned object, an exhaust-gas-purifying catalyst production process comprises the steps of: immersing an alumina powder into a platinum solution, thereby preparing an alumina powder with platinum loaded thereon; thermally treating the platinum-loaded alumina powder at a temperature of 900 to 1200°C in non-oxidizing atmosphere, thereby preparing an alumina powder with platinum fixed thereon; immersing an alumina powder into a rhodium solution, thereby preparing an alumina powder with rhodium loaded thereon; and coating the platinum-fixed alumina powder and the rhodium-loaded alumina powder on a surface of a support substrate. According to the present invention, platinum (Pt) and rhodium (Rh) are loaded on an independent alumina powder, respectively. Then, the independent alumina powders with Pt and Rh loaded respectively thereon are coated on a support substrate. Accordingly, Pt and Rh encounter with a reduced probability. Further, the platinum-loaded alumina powder is further subjected to the thermal treatment in non-oxidizing atmosphere in order to prepare the platinum-fixed alumina powder. In high-temperature atmosphere in which oxygen coexists, platinum loaded on an alumina surface is oxidized to PtO 2 . The resulting PtO 2 is facilitated to diffuse and aggregate by vapor-phase migration. Accordingly, in the fuel-lean or stoichiometric-point atmosphere which includes O 2 in a large amount, platinum exposed to elevated temperatures is sintered to degrade its catalytic capability considerably. In order to avoid the drawback, according to the present invention, the Pt-loaded support is thermally treated in a temperature range of from 900 to 1,200 °C, in non-oxidizing atmosphere after loading a noble metal catalyst ingredient. Thus, as illustrated in Figure 1, a porous support 1 is sintered to contract its pores 10, and the loaded noble metal catalyst ingredient 2 is closely surrounded by the porous support 1. As a result, the platinum catalyst ingredient is unlikely to migrate. Further, the thermal treatment is carried out in non-oxidizing atmosphere, a platinum catalyst ingredient is hardly oxidized to its oxide ( PtO 2 ), and thereby no vapor-phase migration occurs. Accordingly, the platinum catalyst ingredient is virtually inhibited from sintering, and it is primarily subjected to the reaction in which it is surrounded closely by the porous support. As a result, in the resulting exhaust-gas-purifying catalyst, the platinum catalyst ingredient is prevented from migrating, and thereby it is inhibited from sintering even in fuel-lean atmosphere. In order to closely surround the platinum catalyst ingredient by the porous support more effectively during the thermal treatment, it is preferred that the porous support has an average pore diameter in a range of from 1 to 5 nm, further preferably from 2 to 4 nm, before the thermal treatment. When it has an average pore diameter of less than 1 nm, the platinum catalyst ingredient is less likely to be loaded in the pores. Moreover, if such is the case, the pores might be closed by the contraction resulting from the thermal treatment, and thereby the catalytic capability of the resulting exhaust-gas-purifying catalyst might be deteriorated. When it has an average pore diameter of more than 5 nm, the porous support comes to have a reduced surface area and degraded holding ability for the platinum catalyst ingredient or the like. Moreover, if such is the case, it is difficult for the pores to closely surround the platinum catalyst ingredient even when they are contracted by the thermal treatment. Thus, the pores of such a large average diameter allow the platinum catalyst ingredient to migrate. In accordance with the present exhaust-gas-purifying catalyst production process, the platinum catalyst ingredient is closely surrounded by the porous support. As a result, the platinum catalyst ingredient can be inhibited from migrating, and consequently it can be prevented from sintering. Consequently, Pt is closely surrounded by alumina. In conventional exhaust-gas-purifying catalysts with Pt and Rh loaded, however, it has been found that Pt migrates towards Rh and makes alloy therewith. As a result, the sintering develops to deteriorate the catalytic capability of conventional Pt-and-Rh-loaded exhaust-gas-purifying catalysts. On the other hand, according to the present invention, the aforementioned two advantageous actions inhibit Pt and Rh from alloying. Moreover, Pt elements are inhibited from sintering. All in all, according to the present invention, even when the resulting exhaust-gas-purifying catalyst is exposed to elevated temperatures in fuel-lean atmosphere, Pt is securely inhibited from sintering, and Rh is little impaired in terms of its properties (e.g., catalytic and co-catalytic capabilities). In accordance with the present exhaust-gas-purifying catalyst production process, the platinum elements and the rhodium elements are loaded independently. As a result, the platinum elements and the rhodium elements can be kept from making Pt-Rh alloys. Moreover, the platinum elements are closely surrounded by the alumina powder, and accordingly they can be prevented from sintering. All in all, in accordance with the present exhaust-gas-purifying catalyst production process, the noble metal catalyst ingredient can be prevented from sintering. Hence, the present exhaust-gas-purifying catalyst production process enable to securely and easily produce an exhaust-gas-purifying catalyst of superb heat-resistance. According to the present invention, the porous support or the support substrate is alumina which has a large specific surface area. It is especially preferred that the porous support or the support substrate be active alumina which exhibits high heat-resistance and has a large specific surface area. The porous support or the support substrate can be formed as a honeycomb shape, or it can be coated on a surface of refractory substrate which is formed as a pellet shape. In addition, the refractory substrate itself can be formed of the porous support. According to the present invention, the noble metal catalyst ingredient is platinum (Pt) and rhodium (Rh). A loading amount of the noble metal catalyst ingredient can preferably be 0.1% by weight or more, further preferably from 0.5 to 20% by weight, with respect to the weight of the porous support or the support substrate taken as 100%. When the loading amount of the noble metal catalyst ingredient is less than 0.1% by weight, the resulting exhaust-gas-purifying catalyst effects deteriorated CO and HC oxidation reactions, and it particularly effects degraded NO x reduction reactions at low temperatures unfavorably. When the loading amount of the noble metal catalyst ingredient exceeds 20% by weight, such loading not only saturates the catalytic actions of the noble metal catalyst ingredient but also pushes up the production costs, and the resulting exhaust-gas-purifying catalyst is likely to effect degraded NO x reduction reactions at high temperatures. In the description of the present invention, the non-oxidizing atmosphere means atmosphere which is free from the inclusion of oxygen. For example, it can be reducing atmosphere, inert gas atmosphere, and vacuum atmosphere. In the present invention, the porous support or the support substrate can be processed to have the average pore diameter of from 1 to 5 nm by the following process; namely: hydrated alumina is first produced by a pH-swinging process, a coprecipitation process, an alkoxide process, or a compound precipitation process. Then, the resulting hydrated alumina is thermally treated at a temperature of from 400 to 700 °C. In the process of the present invention, when coating the platinum-fixed alumina powder and the rhodium-loaded alumina powder on a surface of a support substrate, they can be coated thereon compositely, or they can be coated thereon independently in two layers. When they are coated thereon independently in two layers, it is preferred that, in order to inhibit Pt from being poisoned by sulfur, the rhodium-loaded alumina powder be disposed at the top layer, and the platinum-fixed alumina powder be disposed at the bottom layer. Moreover, the exhaust-gas-purifying catalyst produced in accordance with the present invention can further include an extra component such as the following metallic oxides: ceria, zirconia, lanthana, and yttria. It is needless to mention that, when the resulting exhaust-gas-purifying catalyst includes the extra component, it operates to produce additional advantageous effects which are inherent to the extra components. BRIEF DESCRIPTION OF THE DRAWINGS A more complete appreciation of the present invention and many of its advantages will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings and detailed specification, all of which forms a part of the disclosure: Figure 1 is a schematic diagram for illustrating changes of a porous support during a thermal treatment step in non-oxidizing atmosphere according to the First Special Embodiment (not claimed); Figure 2 is a graph for illustrating a relationship between thermal treatment temperatures (in air) and average particle diameters of Pt particles in exhaust-gas-purifying catalysts which were produced by a First Special Embodiment (not claimed); Figure 3 is a graph for illustrating a relationship between pore diameters of alumina and temperatures at 50%-HC conversion in exhaust-gas-purifying catalysts which were produced by a Second Special Embodiment (not claimed); Figure 4 is a schematic diagram for illustrating changes of alumina during a thermal treatment step in non-oxidizing atmosphere in a First Preferred Embodiment according to the present invention; Figure 5 is a schematic diagram for illustrating a construction of an exhaust-gas-purifying catalyst produced by the First Preferred Embodiment; and Figure 6 is a schematic diagram for illustrating a construction of an exhaust-gas-purifying catalyst produced by a Second Preferred Embodiment according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED AND SPECIAL EMBODIMENTS Having generally described the present invention, a further understanding can be obtained by reference to the specific preferred embodiments which are provided herein for purposes of illustration only and are not intended to limit the scope of the appended claims; as compared to Special Embodiments, which do not belong to the claimed invention. First Special Embodiment (not claimed) (Alumina Preparation Step) An aluminum nitrate aqueous solution and a sodium aluminate aqueous solution were alternately mixed ten times so that the resulting mixed solution exhibited pH values, 3 and 10, alternately. The aluminum nitrate aqueous solution and the sodium aluminate aqueous solution had a pH value of 1' and 12, respectively. The resulting precipitate was filtered, and washed with distilled water. The resulting powder was calcinated at 500 °C, thereby preparing an alumina powder. The resulting alumina powder had an average pore diameter of 2 nm, and exhibited a sharp pore-diameter distribution. (Noble Metal Catalyst Ingredient Loading Step) The alumina powder was immersed into a predetermined amount of a platinum dinitrodiammine aqueous solution which had a predetermined concentration, and which was acidified by nitric acid. Then, the water content of the aqueous solution was evaporated to dry the alumina powder. Thereafter, the alumina powder was thermally treated at 300 °C for 2 hours, thereby loading Pt thereon. The loading amount of Pt was 2% by weight. (Thermal Treatment Step) The Pt-loaded alumina powder was calcinated at 1,000 °C in vacuum of 10 -1 Pa (10 -3 Torr) for 5 hours. After the calcination, the alumina powder was subjected to an X-ray diffraction analysis. According to the results of this analysis, it was verified that no peak resulting from Pt was appeared in the X-ray diffraction chart, and no Pt elements were crystallized. (Exhaust-Gas-Purifying Catalyst Preparation Step) With respect to 100 parts by weight of the thus calcinated Pt-loaded alumina powder, 45 parts by weight of an aluminum nitrate aqueous solution having a concentration of 40% by weight, 3 parts by weight of an alumina sol having an alumina content of 10% by weight, and 100 parts by weight of water were mixed so as to prepare a slurry. Then, a plurality of honeycomb substrates formed of cordierite were immersed into and taken out of the resulting slurry, and thereafter each of them was blown to blow away the slurry in excess. The honeycomb substrates had a volume of 1.5 liter. Each of the substrates was dried at 120 °C, and thereafter each of them was calcinated at 500 °C. A plurality of exhaust-gas-purifying catalysts were thus produced by the First Special Embodiment of a not claimed exhaust-gas-purifying catalyst production process. Comparative Example No. 1 An alumina powder was prepared by the alumina preparation step of the First Special Embodiment. The resulting alumina powder free from Pt loading was calcinated at 1,000 °C in air for 5 hours. Then, the thus calcinated alumina powder was immersed into a predetermined amount of a platinum dinitrodiammine aqueous solution which had a predetermined concentration, and which was acidified by nitric acid. The water content of the aqueous solution was evaporated to dry the alumina powder. Thereafter, the alumina powder was thermally treated at 300 °C for 2 hours, thereby loading Pt thereon. The loading amount of Pt was 2% by weight. Finally, a plurality of exhaust-gas-purifying catalysts were prepared by subjecting the Pt-loaded alumina powder to the exhaust-gas-purifying catalyst preparation step of the First Special Embodiment. Examination and Evaluation Each of the exhaust-gas-purifying catalysts produced by the First Special Embodiment and Comparative Example No. 1 was subjected to a durability test using a model gas. The model gas simulated an exhaust gas having an air/fuel (A/F) ratio of 18, and included gaseous components in amounts as recited in Table 1 below. Each of the catalysts was exposed to the model gas flow of 25 liter/minute, and degraded at 900 °C for 5 hours. Gaseous Component CO C 3 H 6 NO O 2 CO 2 H 2 O N 2 Content (vol. %) 0.10 0.06 0.25 3.50 11.5 10 Balance Then, each of the catalysts were examined for its HC conversions before and after the durability test. In this HC-conversion examination, another model gas was used which simulated an exhaust gas having an A/F ratio of 14.5 +/- 1, and included gaseous components in amounts as recited in Table 2 below. Note that each of the catalysts were subjected a pretreatment; namely: it was exposed to the model gas which was flowed at a space velocity (SV) of 1.0 x 10 5 /hour, and it was held therein at 500 °C for 20 minutes. Then, each of them was examined for its HC conversions under the following thermal conditions; namely: the temperature of the model gas was reduced by a factor of 5 °C/minutes, and was held for 10 minutes at every 20 °C reduction in order to examine each of them for its HC conversions. The results are recited as temperatures at 50% HC conversion (hereinafter abbreviated to "T50") in Table 3 below. Gaseous Component CO C 3 H 6 NO O 2 CO 2 H 2 O N 2 Content (vol. %) 0.50 0.07 0.28 0.30 14.5 10 Balance Initial After Durability Test 1st Special Embodiment 240 °C 260 °C Comparative Example No. 1 240 °C 330 °C It is apparent from Table 3 that, even after the durability test, the catalysts produced by the First Special Embodiment exhibited T50 which was lower than T50 exhibited by the catalysts produced by Comparative Example No. 1. Thus, the catalysts produced by the First Special Embodiment were found to be superior to the catalysts produced by Comparative Example No. 1 in terms of durability. In addition, each of the catalysts produced by the First Special Embodiment and Comparative Example No. 1 were thermally treated in air for 5 hours at various temperatures, and, after this thermal treatment, it was observed for its Pt particle diameter by an optical microscope. The results of this observation are illustrated in Figure 2. As can be seen from Figure 2, the catalysts produced by the First Special Embodiment had a Pt particle diameter smaller than that of the catalysts produced by Comparative Example No. 1, and their Pt elements were inhibited from sintering. It is believed that this durability difference resulted from the difference in the thermal treatment atmosphere for the Pt-loaded alumina powder. Second Special Embodiment (not claimed) 200 parts by weight of triisopropoxy aluminum was added to 500 milliliters of isopropyl alcohol. The resulting mixture was stirred at 80 °C for 5 hours. Further, 60 milliliters of water was added to the stirred mixture to hydrolyze it, thereby preparing alumina hydrate. Then, the alumina hydrate was subjected to reduced-pressure drying to remove the alcohol. Thereafter, the thus dried alumina hydrate was calcinated at 700 °C for 5 hours, thereby preparing an alumina powder having an average pore diameter of 5 nm. Furthermore, in a manner similar to the Pt-loading and thermal-treatment steps of the First Special Embodiment, Pt was loaded on the alumina powder, and the Pt-loaded alumina powder was thermally treated at 1,000 °C in vacuum of 10 -3 for 5 hours. After this thermal treatment, the thermally-treated alumina powder was subjected to an X-ray diffraction analysis. According to the resulting X-ray diffraction chart, no Pt crystal was appreciated. Finally, in a manner similar to the First Special Embodiment, exhaust-gas-purifying catalysts were prepared. The resulting catalysts were also subjected to the durability test, and were similarly examined for their HC conversions before and after the durability test. The results of this examination are illustrated as T50 with respect to alumina pore diameter in Figure 3. In Figure 3, solid circles () and solid lines specify initial T50 (i.e., T50 before the durability test), and blank circles (○) and dotted lines specify T50 after the durability test. In addition, the thermal treatment condition was varied in order to prepare alumina powders having a variety of average pore diameters which fell in a range of from 0.5 to 10 nm. Likewise, various exhaust-gas-catalysts were prepared with these alumina powders, and were similarly examined for their HC conversions before and after the durability test. The results of this examination are also illustrated in Figure 3. It is understood from Figure 3 that, when the alumina powders had an average pore diameter of from 1 to 5 nm, the resulting exhaust-purifying-gas catalysts exhibited reduced HC-conversion differences before and after the durability test, and that these catalysts were especially superb in terms of durability. First Preferred Embodiment (Alumina Preparation Step) An aluminum nitrate aqueous solution and a sodium aluminate aqueous solution were alternately mixed ten times so that the resulting mixed solution exhibited pH values, 3 and 10, alternately. The aluminum nitrate aqueous solution and the sodium aluminate aqueous solution had a pH value of 1 and 12, respectively. The resulting precipitate was filtered, and washed with distilled water. The resulting powder was calcinated at 500 °C, thereby preparing an alumina powder. The resulting alumina powder had an average pore diameter of 2 nm, and exhibited a sharp pore-diameter distribution. (Platinum Loading Step) The alumina powder was immersed into a predetermined amount of a platinum dinitrodiammine aqueous solution which had a predetermined concentration, and which was acidified by nitric acid. Then, the water content of the aqueous solution was evaporated to dry the alumina powder. Thereafter, the alumina powder was thermally treated at 300 °C for 2 hours, thereby loading Pt thereon. The loading amount of Pt was 2% by weight. (Thermal Treatment Step) The Pt-loaded alumina powder was thermally treated at 1,000 °C in vacuum of 10 -1 Pa (10 -3 Torr) for 5 hours. Thus, an alumina powder with Pt fixed was prepared. Figure 4 schematically illustrates the changes in this thermal treatment step; namely: platinum elements Pt2 are loaded in pores 10 of an alumina powder 1. Since there exists no oxygen, the platinum elements Pt2 are inhibited from migrating in vapor phase. As a result, when the pores 10 are shrunk by the calcination, the platinum elements Pt2 are surrounded by and fixed closely in the alumina powder 1. (Rhodium Loading Step) With respect to 50 parts by weight of active alumina, 50 parts by weight of rhodium chloride aqueous solution having a predetermined concentration was added to mix. Then, the water content of the aqueous solution was evaporated to dry the active alumina. Thereafter, the active alumina was dried at 150 °C for 8 hours, and further it is calcinated at 500 °C for 1 hour, thereby preparing an Rh-loaded active alumina powder. The loading amount of Rh was 0.6% by weight. (Coating Step) 50 parts by weight of the Pt-fixed alumina powder, 50 parts by weight of the Rh-loaded active alumina powder, 60 parts by weight of an aluminum nitrate aqueous solution having a concentration of 40% by weight, 10 parts by weight of an alumina sol having an alumina content of 10% by weight, 40 parts by weight of a cerium oxide-zirconium oxide composite oxide powder, 20 parts by weight of a lanthanum oxide powder, and 80 parts by weight of pure water were mixed and stirred so as to prepare a slurry. Then, a plurality of honeycomb substrates formed of cordierite were immersed into and taken out of the resulting slurry, and thereafter each of them was blown to blow away the slurry in excess. The honeycomb substrates had a volume of 1.5 liter. Each of the substrates was dried at 250 °C for 1 hour, and thereafter calcinated at 500 °C for 1 hour. A plurality of exhaust-gas-purifying catalysts were thus produced by the First Preferred Embodiment of the present exhaust-gas-purifying catalyst production process. Figure 5 schematically illustrates one of the exhaust-gas-purifying catalysts thus prepared; namely: in the exhaust-gas-purifying catalyst, Pt-fixed alumina particles 3, Rh-loaded active alumina particles 4, cerium oxide-zirconium oxide composite oxide particles 5, and lanthanum oxide particles 6 are mixed and coated on a surface of a support substrate. Second Preferred Embodiment (Alumina Preparation Step) 200 parts by weight of triisopropoxy aluminum was added to 500 milliliters of isopropyl alcohol. The resulting mixture was stirred at 80 °C for 5 hours. Further, 60 milliliters of water was added to the stirred mixture to hydrolyze it, thereby preparing alumina hydrate. Then, the alumina hydrate was subjected to reduced-pressure drying to remove the alcohol. Thereafter, the thus dried alumina hydrate was calcinated at 700 °C for 5 hours, thereby preparing an alumina powder having an average pore diameter of 5 nm. (Platinum Loading Step) The alumina powder was immersed into a predetermined amount of a platinum dinitrodiammine aqueous solution which had a predetermined concentration, and which was acidified by nitric acid. Then, the water content of the aqueous solution was evaporated to dry the alumina powder. Thereafter, the alumina powder was thermally treated at 300 °C for 2 hours, thereby loading Pt thereon. The loading amount of Pt was 2% by weight. (Thermal Treatment Step) The resulting Pt-loaded alumina powder was thermally treated at 1,000 °C in a helium gas for 5 hours. The helium gas included H 2 in an amount of 10% by volume. Finally, the rhodium-loading step and the coating step were carried out in the same manner as those of the First Preferred Embodiment, and thereby exhaust-gas-purifying catalysts were prepared by the Second Preferred Embodiment. Third Preferred Embodiment 50 parts by weight of the Pt-fixed alumina powder which was identical with the one prepared by the First Preferred Embodiment, 30 parts by weight of an aluminum nitrate aqueous solution having a concentration of 40% by weight, 5 parts by weight of an alumina sol having an alumina content of 10% by weight, 20 parts by weight of a cerium oxide-zirconium oxide composite oxide powder, 10 parts by weight of a lanthanum oxide powder, and 40 parts by weight of pure water were mixed and stirred so as to prepare a first slurry. Then, a plurality of honeycomb substrates formed of cordierite was immersed into and taken out of the resulting first slurry, and thereafter each of them was blown to blow away the first slurry in excess, thereby forming a Pt-fixed coating layer. The honeycomb substrates had a volume of 1.5 liter. Further, 50 parts by weight of the Rh-loaded active alumina powder which was identical with the one prepared by the First Preferred Embodiment, 30 parts by weight of an aluminum nitrate aqueous solution having a concentration of 40% by weight, 5 parts by weight of an alumina sol having an alumina content of 10% by weight, 20 parts by weight of a cerium oxide-zirconium oxide composite oxide powder, 10 parts by weight of a lanthanum oxide powder, and 40 parts by weight of pure water were mixed and stirred so as to prepare a second slurry. Furthermore, each of the honeycomb substrates having the Pt-fixed coating layer was immersed into and taken out of the resulting second slurry, and thereafter each of them was blown to blow away the second slurry in excess. Each of the substrates was dried 80 °C for 20 minutes, and thereafter each of them was calcinated at 600 °C for 1 hour, and thereby exhaust-gas-purifying catalysts were prepared by the Third Preferred Embodiment. Figure 6 schematically illustrates one of the exhaust-gas-purifying catalysts thus prepared; namely: in the exhaust-gas-purifying catalyst, a first coating layer 70 is disposed on a surface of a support substrate, and a second coating layer 71 is disposed on the first coating layer 70. The first coating layer 70 includes Pt-fixed alumina particles 3, cerium oxide-zirconium oxide composite oxide particles 5, and lanthanum oxide particles 6. The second coating layer 71 includes Rh-loaded active alumina particles 4, cerium oxide-zirconium oxide composite oxide particles 5, and lanthanum oxide particles 6. Forth Preferred Embodiment Except that the Pt-fixed alumina powder which was identical with the one prepared by the Second Preferred Embodiment, and the Rh-loaded active alumina powder which was identical with the one prepared by the First Preferred Embodiment were employed, and except that the coating step of the Third Preferred Embodiment was followed, a Forth Preferred Embodiment of the present exhaust-gas-purifying catalyst production process was carried out to prepare exhaust-gas-purifying catalysts. Comparative Example No. 2 100 parts by weight of an active alumina powder, 60 parts by weight of an aluminum nitrate aqueous solution having a concentration of 40% by weight, 10 parts by weight of an alumina sol having an alumina content of 10% by weight, 40 parts by weight of a cerium oxide powder, and 80 parts by weight of pure water were mixed and, stirred so as to prepare a slurry. Then, a plurality of honeycomb substrates formed of cordierite were immersed into and taken out of the resulting slurry, and thereafter each of them was blown to blow away the slurry in excess. The honeycomb substrates had a volume of 1.5 liter. Each of the substrates was dried at 80 °C for 20 minutes, and thereafter calcinated at 600 °C for 1 hour, thereby preparing honeycomb substrates with a coating layer formed thereon. Each of the honeycomb substrates with the coating layer formed thereon was immersed into a predetermined amount of a platinum dinitrodiammine aqueous solution which had a predetermined concentration, and which was acidified by nitric acid. After evaporating the water content of the aqueous solution to dry each of the substrates, each of them was further dried at 250 °C, thereby loading Pt thereon. The loading amount of Pt was 1.5 grams with respect to 1 liter of the substrate. Moreover, each of the honeycomb substrates with Pt loaded thereon was immersed into a rhodium chloride aqueous solution having a predetermined concentration. After evaporating the water content of the aqueous solution to dry each of the substrates, each of them was further dried at 250 °C, thereby loading Rh thereon. The loading amount of Rh was 0.3 grams with respect to 1 liter of the substrate. A plurality of exhaust-gas-purifying catalysts were thus produced by Comparative Example No. 2. Examination and Evaluation The five kinds of the exhaust-gas-purifying catalysts produced by the First, Second, Third and Forth Preferred Embodiments, and by Comparative Example No. 2 were subjected to a durability test; namely: the catalysts were respectively accommodated in a catalyst converter. Each of the catalyst converters was disposed in an exhaust line of an engine.. The engine had a displacement of 2 liters. While greatly fluctuating the air-fuel ratio of air-fuel mixture being supplied to the engine around the stoichiometric point and keeping the catalyst bed temperature at 1,000 °C, the engine was driven for 50 hours to degrade the catalysts. For instance, the air-fuel ratio was fluctuated by +/- 1 with respect to the stoichiometric point (e.g., A/F = 14.6 +/- 1). After the durability test, each of the catalysts were examined for its temperatures at 50% HC, CO and NO x conversions. The results of this examination are summarized in Table 4 below. The temperatures at 50% HC, CO and NO x conversions were determined as follows; namely: the air fuel ratio of air-fuel mixture was fixed at the stoichiometric point (e.g., A/F 14.6), and an inlet temperature was increased in a range of from 240 °C to 440 °C until the catalysts exhibited 50% HC, CO and NO x conversions. The inlet temperatures at which the catalysts exhibited 50% HC, CO and NO x conversions were taken as the temperatures at 50% HC, CO and NO x conversions. The cross-conversions were determined as follows; namely: while the air fuel ratio of air-fuel mixture was fluctuated in a range of from 13.5 to 15.0 and an inlet temperature was fixed at 400 °C, the catalysts were examined for HC, CO and NO x conversions. When the CO conversion and the NO x conversion crossed (i.e., when they are equal) at a certain A/F ratio, the CO conversion and the NO x conversion are taken as the CO-NO x conversion, and HC conversion was measured at the A/F ratio. 50% Conversion Temp. ('C) Cross-Conversion (%) HC CO NOx CO-NOx HC 1st Pref. Embodiment 325 324 327 89 94 2nd Pref. Embodiment 327 321 326 90 95 3rd Pref. Embodiment 332 335 328 90 93 4th Pref. Embodiment 321 325 324 89 92 Comparative Ex. No. 2 358 356 349 80 85 As summarized in Table 4, after being subjected to the durability test, the catalysts produced by the First through Forth Preferred Embodiments exhibited lower 50% conversion temperatures and higher cross conversions than those produced by Comparative Example No. 2 did. Thus, it was appreciated that these catalysts were superb in terms of durability. Specifically, in these catalysts, the Pt elements and the Rh elements were disposed so as to separate from each other, and accordingly the Pt elements were accordingly inhibited from sintering. As a result, the durability of these catalysts was upgraded evidently.

Claims: A process for producing an exhaust-gas-purifying catalyst, comprising the steps of: immersing an alumina powder into a platinum solution, thereby preparing an alumina powder with platinum loaded thereon; thermally treating the platinum-loaded alumina powder at a temperature of 900 to 1200°C in non-oxidizing atmosphere, thereby preparing an alumina powder with platinum fixed thereon; immersing an alumina powder into a rhodium solution, thereby preparing an alumina powder with. rhodium loaded thereon; and coating the platinum-fixed alumina powder and the rhodium-loaded alumina powder on a surface of a support substrate. The process according to Claim 1, wherein said step of immersing an alumina powder into a platinum solution further includes a step of selecting an alumina powder having an average pore diameter which falls in a range of from 1 to 5 nm. The process according to Claim 1 or 2, wherein, in said step of immersing an alumina powder into a platinum solution, platinum is coated in an amount of 0.1% by weight or more with respect to the weight of alumina powder taken as 100%. The process according to any of Claims 1 to 3, wherein, in said step of immersing an alumina powder into a rhodium solution, rhodium is coated in an amount of 0.1% by weight or more with respect to the weight of alumina powder taken as 100%. The process according to any of Claims 1 to 4, wherein, in said step of coating, the platinum-fixed alumina powder and the rhodium-loaded alumina powder are coated compositely. The process according to any of Claims 1 to 4, wherein, in said step of coating, the platinum-fixed alumina powder and the rhodium-loaded alumina powder are coated independently in two layers including a top layer and a bottom layer, the top layer includes the rhodium-loaded alumina powder, and the bottom layer includes the platinum-fixed alumina powder. The process according to any of Claims 1 to 6, wherein said thermally-treating step is carried out in at least one atmosphere selected from the group consisting of reducing atmosphere, inert gas atmosphere, and vacuum atmosphere.

Inventors: SUZUKI JUJI, TAGUCHI NORIO, SUZUKI, JUJI, TAGUCHI, NORIO
Applicants: TOYOTA MOTOR CO LTD, TOYOTA JIDOSHA KABUSHIKI KAISHA
Application year: 1995
Main classification: B01J37
IPC codes: B01D53, B01J37, B01J23, B01J27