Exhaust gas purifying catalyst and exhaust gas purifying method

Abstract: An exhaust gas purifying catalyst is formed of a first powder formed of porous particles supporting rhodium (Rh), and a second powder formed of porous particles supporting platinum (Pt) and a nitrogen oxides (NOx)-adsorbing material. The second powder and the first. powder are present in a mixed state. In this catalyst, Rh is supported apart from Pt and the NOx adsorbent, so that reduction of the oxidizing performance of Pt is prevented, and so that the problems of poor compatibility between Rh and the NOx adsorbent is eliminated. The catalyst therefore maintains high NOx removing rate not only during an initial period but also after the endurance test.

Description: BACKGROUND OF THE INVENTION 1. Field of the invention The present invention relates to an exhaust gas purifying catalyst and an exhaust gas purifying method for purifying exhaust gas from an internal combustion engine of a motor vehicle or the like and, more particularly, to an exhaust gas purifying catalyst and an exhaust gas purifying method capable of efficiently removing nitrogen oxides (Nox) by reduction, from exhaust gas that contains an amount of oxygen in excess of the amount of oxygen required for complete oxidation of the reducing components of the exhaust gas, such as carbon monoxide (CO), hydrogen gas (H 2 ), hydrocarbon (HC) and the like. 2. Description of the Related Art Many conventional motor vehicles employ three-way catalysts that purify exhaust gas by simultaneously oxidizing CO and HC and reducing NOx in exhaust gas at a theoretical air-fuel ratio (stoichiometric ratio). In a widely known three-way catalyst of this type, a heat-resistant base member formed of, for example, cordierite, carries thereon a porous support layer formed of γ -alumina, and the porous support layer supports catalytic noble metals such as platinum (Pt), rhodium (Rh) and the like. Recently, carbon dioxide (CO 2 ) in exhaust gas from internal combustion engines of motor vehicles and the like has become an issue in view of protection of global environments. A promising technology for reducing the amount of CO 2 emission from an internal combustion engine is a lean burn system, in which combustion is performed at a lean air-fuel ratio with an excessive amount of oxygen. The lean burn system reduces fuel consumption due to improved fuel efficiency and, therefore, reduces the amount of CO 2 produced by combustion. Since the conventional three-way catalysts achieve simultaneous oxidation of CO and HC and reduction of NOx in exhaust gas when the air-fuel ratio is substantially the stoichiometric ratio, the conventional three-way catalysts fail to sufficiently remove NOx by reduction in lean-burnt exhaust gas, which contains an excessive amount of oxygen. Therefore, there has been a need for development of a catalyst and an exhaust gas purifying system capable of removing NOx even in an excessive-oxygen atmosphere. The present applicant proposed in, for example, Japanese Patent Application Laid-Open No. Hei 5-317652, an exhaust gas purifying catalyst in which an alkaline earth metal, such as barium (Ba) or the like, and platinum (Pt) are supported by a porous support formed of alumina or the like. Using the exhaust gas purifying catalyst, NOx can be efficiently removed from lean-burnt exhaust gas from a lean burn system if the air-fuel ratio is controlled so that the air-fuel ratio shifts from a lean side to a stoichiometric/rich side in a pulsed manner. NOx is adsorbed by the alkaline earth metal (NOx adsorbent) on the lean side, and reacts with reducing components, such as HC, CO and the like, on the stoichiometric/rich side. It has been found that the aforementioned NOx-removing process using the exhaust gas purifying catalyst proceeds in three steps: the first step in which NO in exhaust gas is oxidized into NOx; the second step in which NOx is adsorbed by the NOx adsorbent; and the third step in which NOx released from the NOx adsorbent is reduced on the catalyst. However, in the conventional exhaust gas purifying catalysts, particle growth of platinum (Pt) occurs in a lean atmosphere, thereby reducing the number of catalytic reaction points. Therefore, the reactivity in the first and third steps inconveniently decrease. Rhodium (Rh) is known as a catalytic noble metal that has a less tendency to undergo particle growth in a lean atmosphere. However, the oxidizing capacity of Rh is considerably lower than that of Pt. Use of a combination of Pt and Rh may be considered. It is known that co-presence of Pt and Rh reduces the particle grow of Pt. With regard to combined use of Pt and Rh, it has been found that as the amount of Rh contained increases, the oxidizing capacity of Pt decreases, probably because Rh covers Pt surfaces. Therefore, as the an content increases, the reactivity of oxidation of NO into NOx in the first step decreases and the NOx-adsorbing rate in the second step also decreases. Another problem with Rh is low compatibility with NOx adsorbents. Co-presence of Rh and a NOx adsorbent results in insufficient performance of the NOx adsorbent and Rh. In addition, sulfur (S) components contained in fuel are oxidized into SO 2 , which is further oxidized on the catalyst into sulfates. If sulfates react with the NOx adsorbent, the NOx adsorbing capacity of the NOx adsorbent is lost, thereby impeding the removal of NOx by reduction. This undesired phenomenon is generally termed sulfur-poisoning of NOx adsorbent. SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to prevent a durability reduction due to the particle growth of platinum (Pt) by using rhodium (Rh) in addition to Pt and improve durability by preventing reduction of the NOx adsorbing and releasing performance of the NOx adsorbent. According to one aspect of the invention, there is provided an exhaust gas purifying catalyst including a base member, and a coating layer formed on a surface of the base member. The coating layer is formed of a first powder formed of porous particles supporting rhodium and a second powder formed of porous particles supporting a nitrogen oxides-adsorbing material. The first powder and the second powder are present in a mixed state. Platinum is supported in the coating layer. The concentration of platinum supported in a surface layer portion being higher than the concentration of platinum supported in an internal portion. At least one of the first powder and the second powder may support at least one element selected from the group consisting of cobalt, iron and nickel. The second powder may support 1-10% by weight of rhodium relative to an amount of platinum. A hydrocarbon-adsorbing adsorbent may be provided adjacent to the first powder. The porous particles of the first powder may serve as the hydrocarbon-adsorbing adsorbent. The HC-adsorbing adsorbent may exist at an interface between the first powder and the second powder. According to another aspect of the invention, there is provided an exhaust gas purifying method. In a step of the method, a catalyst as described above is disposed in an exhaust gas passage. In another step, the nitrogen oxides-adsorbing material is allowed to adsorb nitrogen oxides in a lean atmosphere in which an excessive amount of oxygen is present. In still another step, the lean atmosphere is temporarily changed to a stoichiometric-rich atmosphere so as to cause reduction of nitrogen oxides released from the nitrogen oxides-adsorbing material. BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and further objects, features and advantages of the present invention will become apparent from the following description of preferred embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein: Fig. 1 is an illustration of the structure of a coating layer in an exhaust gas purifying catalyst of Example 1 of the invention; Fig. 2 illustrates the structure in an interior portion of a coating layer shown in Fig. 1; and Fig. 3 illustrates the structure in a surface portion of the coating layer shown in Fig 1. DETAILED DESCRIPTION OF THE INVENTION The present inventors have developed an exhaust gas purifying catalyst, which catalyst is not comprised by the present invention, wherein rhodium (Rh) is present in a first powder, and platinum (Pt) and the nitrogen oxides adsorbing material (NOx adsorbent) are present in a second powder, and the first powder and the second powder are in a mixed state. That is, Pt and the NOx adsorbent are supported adjacent to each other, and Rh and Pt are supported apart from each other. The particle growth of Rh in a lean atmosphere is remarkably less than that of Pt. Therefore, the presence of Rh improves durability. Furthermore, since Rh is supported apart from the NOx adsorbent, the problem of poor compatibility between the two components is eliminated, so that performance deterioration of the NOx adsorbent and Rh is prevented. Furthermore, since Rh is separately supported, hydrogen, having a high reducing power, is produced from hydrocarbon (HC) and H 2 O in exhaust gas by Rh (water vapor reforming reaction). Hydrogen thus produced considerably contributes to the reduction of NOx and the desorption of sulfur oxides (SOx) from the NOx adsorbent which is subjected to sulfur-poisoning. Therefore, the reduction of Nox during rich pulses increases, and the sulfur-poisoning considerably decreases. Although the water vapor reforming activity of Rh is normally reduced by the presence of the NOx adsorbent, the exhaust gas purifying catalyst prevents this phenomenon and therefore allows water vapor reforming reaction by Rh to a maximum-possible level since Rh and the NOx adsorbent are supported apart from each other. Therefore, if this exhaust gas purifying catalyst is actually exposed to a lean atmosphere (lean burnt exhaust gas) having an excessive amount of oxygen, nitrogen monoxide (NO) is oxidized into NOx on Pt in the second powder, and NOx is rapidly adsorbed by the NOx adsorbent supported adjacent to Pt. Since Rh is supported apart from Pt, the inhibition of oxidizing capacity of Pt by Rh is prevented, so that NO is efficiently converted into NOx. Furthermore, since Rh is also apart from the NOx adsorbent, the deterioration of NOx adsorbing activity is prevented. HC and CO in exhaust gas react with oxygen present in an excessive amount by the catalytic reaction of Pt and Rh. Thus, HC and CO are readily removed by oxidation. In stoichiometric-rich atmospheres, NOx is released from the NOx adsorbent, and then reacts with HC and CO in exhaust gas by the catalytic reaction of Pt and Rh. Thus, NOx is removed by reduction into nitrogen gas (N 2 ). The NOx reducing performance is improved by the hydrogen produced by the water vapor reforming reaction shown as formula (1). The reaction of the formula (1) occurs by the rhodium(Rh), and when a first powder formed of zirconia supporting rhodium(Rh) is utilized, the water vapor reforming reaction is further accelerated. CnHm + 2nH 2 O --> nCO 2 + (m/2 + 2n)H 2 In this exhaust gas purifying catalyst it is preferable that at least one of the first powder and the second powder carry at least one element selected from cobalt (Co), iron (Fe) and nickel (Ni). These elements accelerate aqueous gas shift reaction expressed in formula (2), thereby accelerating generation of hydrogen. CO + H 2 O → CO 2 + H 2 With its strong reducing power, hydrogen reduces NOx, so that the exhaust gas purifying catalyst maintains a high NOx reducing performance even after the endurance test. Furthermore, hydrogen occurring in the vicinity of the Nox adsorbent reduces sulfates taken up by the NOx adsorbent, thereby reducing the degradation of the NOx adsorbent by sulfur-poisoning. The amount of the at least one of Co, Fe and Ni supported in the exhaust gas purifying catalyst is preferably in the range of 0.01-2.0 moles relative to 120 g of the porous particles, whether a single element or two or more elements are selected. If the amount is less than 0.01 mole relative to 120 g, no substantial effect is achieved. If the amount is greater than 2.0 moles relative to 120 g, no further improvement is achieved and the reaction of the catalytic noble metal may be reduced. The at least one element selected from Co, Fe and Ni may be supported on either the porous particles of the first powder or the porous particles of the second powder, and may also be supported on the porous particles of both powders. The porous particles carrying the at least one elements of Co, Fe and Ni may preferably support a promoter of at least one of silicon (Si) and magnesium (Mg). Provision of the promoter will further promote the hydrogen-producing reactions. The amount of the promoter supported on the porous particles is preferably within the range of 0.005-2.0 moles relative to 120 g of the porous particles. If the amount is less than 0.005 mole relative to 120 g, no substantial effect is achieved. If the amount is greater than 2.0 moles relative to 120 g, no further improvement is achieved. A particularly preferable range of the amount of the promoter is 0.01-0.5 mole relative to 120 g of the porous particles. It is also preferable that the second powder carry 1-10% by weight of Rh relative to the amount of Pt. If a small amount of Rh is thus supported in the second powder, adjacent to Pt, the desirable effect of Rh, that is, reduction of Pt particle growth, becomes dominant over the undesired effect of Rh, that is, reduction of the oxidizing performance of Pt, thereby further improving durability. If the amount of Rh supported in the second powder is less than 1 % by weight relative to the weight of Pt, no substantial effect of Rh is achieved. If the amount exceeds 10% by weight, the reduction of the oxidizing performance of Pt by Rh becomes significantly great, so that the aforementioned reaction in the first step of the NOx-removing process becomes slow. The NOx removing performance therefore decreases. It is also preferred that Pt and Rh be present in the form of a solid solution in the second powder. It is speculated that if Pt and Rh forms a solid solution, the aforementioned desirable effect of Rh will more likely surpass the undesired effect of Rh. Through closer investigation of the acting mechanism of the catalyst, it has become clear that in a rich atmosphere, reaction between water vapor and HC in exhaust gas is occured on the first powder, producing hydrogen, which in turn contributes to the reduction of NOx. That is, the water vapor reforming reaction with HC in exhaust gas occurs on the first powder, thereby producing hydrogen, which in turn reduces NOx released from the NOx adsorbent in a rich atmosphere. Therefore, the aforementioned reaction in the third step of the NOx-removing process is accelerated. In some cases, sulfur-poisoning of NOx adsorbent occurs, wherein Sox in exhaust gas react with the NOx adsorbent to produce sulfuric acid salts whereby the NOx adsorbing capacity is lost. However, due to the production of hydrogen, SOx in exhaust gas is reduced so that the sulfur-poisoning is considerably prevented. Furthermore, the sulfur-poisoned NOx adsorbent is also reduced by hydrogen, so that the NOx adsorbing capacity is recovered. Therefore, high NOx adsorbing capacity of the NOx adsorbent is maintained, so that the reaction in the third step is accelerated. The reaction between water vapor and HC, that is, a' hydrogen-producing reaction, hardly occurs in a lean atmosphere containing an excessive amount of oxygen because there is almost no HC left in the lean burnt exhaust gas. In a lean atmosphere, oxygen present in an excessive amount oxidizes almost the entire amount of HC into H 2 O and CO 2 . If a little amount of hydrogen is produced, hydrogen immediately reacts with oxygen present in an excessive amount to produce H 2 O. Therefore, the aforementioned hydrogen-producing reactions are not fully enjoyed in purifying exhaust gas from a system, such as a lean burn engine, wherein lean atmospheres occur in most operating conditions. Therefore, it is preferred that a HC-adsorbing adsorbent be provided adjacent to the first powder. Thereby, a large amount of HC present in a rich atmosphere is adsorbed to the HC-adsorbing adsorbent. In a lean atmosphere, HC is released from the HC-adsorbing adsorbent, so that HC reacts with water vapor on the first powder, by reaction of Rh, to produce hydrogen. Thus, provision of the HC-adsorbing adsorbent enables production of hydrogen even in a lean atmosphere, so that NOx is reduced, thereby improving the NOx removing performance. Furthermore, sulfur-poisoned NOx adsorbent and, further, SOx are also reduced by hydrogen, so that the sulfur-poisoning of the NOx adsorbentis eliminated and the NOx adsorbing capacity is recovered. In addition, further sulfur-poisoning is prevented. Therefore, the NOx removing performance is significantly improved. A representative example of the HC-adsorbing adsorbent is zeolite. There are various types of zeolites, such as Y type, A type, X type, ZSM-5, silicalite, mordenite, ferrierite, and the like. All of these types have high HC-adsorbing capacity. Among them, Y-type zeolite, ZSM-5, silicalite and mordenite have particularly high HC-adsorbing capacity. Therefore, it is preferred to use a zeolite selected from those zeolites. To increase the hydrogen production rate by increasing the rate of reaction between HC and water vapor, it is necessary that the HC-adsorbing absorbent, which supplies HC in lean atmospheres, be adjacent to the first powder, which offers reaction active points for the reaction between HC and water vapor. This requirement may be achieved by employing first powder in which porous particles are served as a HC-adsorbing adsorbent. Such first powder can be formed by, for example, supporting or fixing at least Rh on porous particles formed of zeolite. The thus-produced first powder will adsorb HC in rich atmospheres, and releases HC in lean atmospheres, which immediately reacts with water vapor on the first powder. Therefore, the reaction between HC and water vapor readily occurs on the first powder to produce hydrogen not only in rich atmospheres but also in lean atmospheres. The aforementioned requirement may also be achieved by disposing a HC adsorbent at an interface between the first powder and the second powder. For example, a HC-adsorbing adsorbent in the form of particles is mixed with the first powder and the second powder, to form a catalyst. In this manner, the HC-adsorbing adsorbent is disposed at the interface between the first powder and the second powder, so that HC is adsorbed to the HC-adsorbing adsorbent in rich atmospheres, and is released therefrom in lean atmospheres to immediately react with water vapor on the first powder to produce hydrogen. In still another possible manner, a HC-adsorbing adsorbent-formed coating layer is provided on a monolithic base member, the first powder is supported onto the coating layer. In some cases, such a coating layer may be formed from the first powder and HC-adsorbing adsorbent powder. In the thus-produced catalyst, HC is adsorbed to the coating layer in rich atmospheres, and is released therefrom in lean atmospheres and reacts with water vapor on the first powder. Therefore, the reaction between HC and water vapor readily occurs on the first powder to produce hydrogen not only in rich atmospheres but also in lean atmospheres. In an example method for producing an exhaust gas purifying catalyst containing such a HC-adsorbing adsorbent, the first powder is prepared by supporting or fixing Rh onto a HC-adsorbing adsorbent powder. The first powder is mixed with the second powder and the mixture is formed into pellets, thereby producing a desired catalyst in the form of pellets. In another example method, a monolithic base or a honeycomb base formed of a metal foil is coated with a slurry containing a mixture of the first and second powders as a main component, and the coated base is baked, thereby producing a desired catalyst. Furthermore, a mixture of the first powder containing no such HC-adsorbing adsorbent, the second powder and a HC-adsorbing adsorbent powder, may be formed into pellets or a coating layer. Further, After a monolithic base is coated with a HC-adsorbing adsorbent, a mixture of the first powder and the second powder may be supported on the coating layer. To support the first powder and second powder on the HC adsorbent, various methods may be employed, for example: a deposition method in which zeolite is suspended in an aqueous solution of a zirconium salt or an aluminum salt, and an alkali is added to shift the pH to a base side so that precipitation occurs on the zeolite; a sol-gel method in which an alkox1de of zirconium or aluminum is hydrolyzed so that the hydroxide of zirconium or aluminum is supported on zeolite, and the like Normally, exhaust gas purifying catalysts are provided in the form of a honeycomb, wherein a cordierite or metal-made honeycomb base is coated with a support material, such as alumina, and the coating layer supports a catalytic metal or a NOx adsorbent. A conventional mainstream method for supporting a noble metal in a catalyst is an adsorption-support method in which a base having a coating layer is dipped into an aqueous solution of a noble metal compound, and then pulled up, dried and baked. In the adsorption-support method, noble metal ions in the aqueous solution are fixed in a surface portion of the coating layer, so that substantially no noble metal ions are contained in the aqueous solution that penetrates into an interior portion. Therefore, noble metal is supported only in a surface layer of the coating layer. This is favorable in view of catalytic reactions. However, in the above-described catalyst wherein Rh and the NOx adsorbent are supported apart from each other, the noble metal and the NOx adsorbent are each supported on powder, and the powder is applied as a coating onto a base to form a desired catalyst. Therefore, the noble metal in this separate-support catalyst is supported uniformly in the entire coating layer. Since noble metal supported in an interior portion of the coating layer may not be fully used, it is desirable to provide a measure for more effective use of noble metal supported in the separate-support catalyst. Therefore, in the exhaust gas purifying catalyst according to the present invention including a base member, a coating layer formed on a surface of the base member, and Rh, Pt and an NOx adsorbent that are supported in the coating layer, the coating layer is formed of a mixture of a first powder formed of porous particles carrying Rh and a second powder formed of porous particles carrying Pt and the NOx adsorbent, and the concentration of Pt supported in a surface portion of the coating layer is higher than that in an interior portioii of the coating layer. With the concentration of Pt supported in a surface portion being higher than that in an interior portion, the probability of contact between Pt and exhaust gas components increases, so that reaction efficiency increases and, therefore, the exhaust gas purifying performance further improved. More specifically, in a lean atmosphere, NO approaches a surface of the catalyst, and becomes oxidized by Pt present in a surface layer into NOx, and enters an interior portion of the catalyst, where it is adsorbed by the NOx adsorbent. At the time of a rich pulse, hydrogen is produced by the water vapor reforming reaction with Rh, and NOx is released from the NOx adsorbent. When NOx passes through the surface layer toward the surface, NOx reacts with hydrogen by the catalytic reaction of Pt. Thus, NOx is reduced to N 2 , and N 2 is discharged. Therefore, the efficiency of using HC at the time of rich pulses further improves, and the NOx removing performance further improves. Although SOx may strongly bind with the NOx adsorbent, such SOx is released by reduction with hydrogen, so that the NOx adsorbent regains the original NOx adsorbing capacity. Therefore, the NOx removing performance further improves. The amount of Pt supported in a surface portion of the coating layer is preferably in the range of 0.1-10 g and, more preferably, in the range of 0.1-2 g, relative to 120 g of the porous particles in the entire catalyst. If the amount of Pt is less than 0.1 g relative to 120 g of the porous particles, no substantial effect of a higher Pt concentration in the surface portion is achieved. If the amount of Pt is higher than 2 g, no further significant improvement of the effect may not be achieved and, in some cases, a cost problem may result. The amount of Pt supported in the entire catalyst may be approximately equal to the amount of Pt supported in conventional catalysts, thereby preventing a considerable cost increase. For example, the amounts of Pt supported in the second powder and the surface layer may be in the range of 0.1-10 g in total relative to 120 g of the porous particles in the entire catalyst, and may be varied within the range of 0-10 g and the range of 10-0 g, respectively. Rh may be supported not only in the first powder but also in a surface layer of the coating layer. In this case, the amount of Rh supported in the surface layer is preferably not more than 10% relative to the weight of Pt supported in the surface layer. If the amount of Rh in the surface layer exceeds 10%, the oxidizing performance of Pt supported in a higher concentration in the surface layer decreases, so that the NOx removing performance decreases. The NOx adsorbent may also be supported in a surface layer of the coating layer. In this case, the NOx adsorbent is preferably an alkali metal achieving a high NOx adsorbing capacity in a small amount. The amount of the NOx adsorbent in the surface layer is preferably in the range of 0-5 moles and, more preferably, in the range of 0.1-0.5 mole, relative to 120 g of the porous particles in the entire catalyst. If the amount of the NOx adsorbent in the surface layer is excessively large, the exhaust gas purifying performance of the precious metal may significantly decreases. To produce an exhaust gas purifying catalyst as described above, the first powder carrying Rh and the second powder carrying Pt and the NOx adsorbent are mixed, and a slurry containing the mixture as a main component is applied as a coating onto a cordierite or metal foil-made honeycomb base. The coated honeycomb base is baked, thereby forming coating layers. The baked honeycomb base is dipped into and pulled up from an aqueous solution containing at least Pt, and then dried and baked, thereby producing a desired catalyst. When the aqueous solution containing Pt contacts the coating layer, Pt ions in the solution adsorbs mainly to a surface layer of the coating layer, and substantially no Pt ion remains in the solution penetrating into an interior portion. Therefore, Pt is supported in the surface layer in a concentrated manner, so that the concentration of Pt supported in the surface layer becomes higher than that in the interior portion. In the exhaust gas purifying catalyst of the invention, HC and co are oxidized on Pt in lean atmospheres containing excessive amounts of oxygen. Simultaneously, there occur the first step wherein NO in exhaust gas is oxidized into NOx and the second step wherein NOx is adsorbed by the NOx adsorbent. Since Pt and the NOx adsorbent are supported adjacent to each other and Rh is supported apart from Pt, the problem of a reduction in the oxidizing performance of Pt due to the presence of Rh in proximity does not arise. Therefore, the first and second steps smoothly proceed. Upon a temporary change to a stoichiometric-rich atmosphere, NOx is released from the NOx adsorbent and reacts with HC and CO in exhaust gas by the catalytic reactions of Pt and Rh. NOx, and HC and CO are thus removed by reduction and oxidation. In the exhaust gas purifying catalyst of the invention, NO is oxidized into NO 2 on Pt in the second powder, and NOx is rapidly adsorbd by the NOx adsorbent supported adjacent to Pt, in a lean atmosphere containing an excessive amount of oxygen. Since Pt and Rh are supported apart from each other, inhibition of the oxidizing performance of Pt by Rh is prevented and, therefore, NO is smoothly converted into NO 2 . Furthermore, since the NOx adsorbent is supported apart from Rh, the deterioration of the NOx removing performance is prevented. Therefore, NOx is smoothly adsorbed by the NOx adsorbent and thereby prevented from being released to outside. HC and CO in exhaust gas react with oxygen present in an excessive amount by catalytic reactions of Pt and Rh. HC and CO are thus readily removed by oxidation. When the atmosphere is changed to a stoichiometric-rich atmosphere, NOx is released from the NOx adsorbent, and reacts with HC and CO in the exhaust gas by the catalytic reactions of Pt and Rh and, in addition, reacts with hydrogen produced on the second powder, thereby turning into N 2 . Thus, NOx is removed by reduction. Even if Pt particle growth occurs so that the reducing power decreases, the great reducing power of Rh still efficiently removes NOx by reduction. HC in exhaust gas adsorbs to and accumulates on the HC-adsorbing adsorbent in a stoichiometric-rich atmosphere. When the atmosphere becomes a lean atmosphere containing an excessive amount of oxygen, HC is released from the HC-adsorbing adsorbent, and reacts with water vapor to produce hydrogen, which in turn reduces NOx and, further, reduces the sulfur-poisoned NOx adsorbent. Therefore, the NOx adsorbing capacity is recovered. Since SOx is reduced by hydrogen, further sulfur-poisoning of the NOx adsorbent is prevented. Therefore, the exhaust gas purifying catalyst of the invention is able to produce hydrogen not only in rich atmospheres but also in lean atmospheres, thereby considerably improving the NOx removing performance. The porous particles, whether for the first powder or the second powder, may be selected from alumina, silica, titania, zirconia, silica-alumina, zeolite and the like. Any one or more of these materials may be selected. If more than one materials are selected, they may be used in a mixture or a composite form. In view of heat resistance, and the good compatibility of zirconia with Rh, it is preferred that zirconia is used for the first powder and alumina is used for the second powder. If zirconia is used for the first powder, the water vapor reforming reaction of Rh remarkably improves. The particle diameter of the porous particles of the first and second powders is preferably within the range of 1-100 µm. If the particle diameter is smaller than 1 µm, it becomes difficult to achieve the effect of the separate supporting of Rh and Pt. If the particle diameter is larger than 100 µm, the interreaction between the first powder and the second powder is inconveniently reduced. The amount of Rh supported in the first powder is preferably within the range of 0.05-20 g relative to 120 g of the porous particles. If the amount of Rh supported is less than 0.05 g relative to 120 g of the porous particles, the durability deteriorates. If it is more than 20 g relative to 120 g, no further improvement of the effect is achieved and a cost increase may result. A particularly preferred range of the amount of Rh supported in the first powder is 0.1-10 g relative to 120 g of the porous particles. It is also possible to support palladium (Pd), iridium (Ir) or the like in the first powder, in addition to Rh. The amount of such material may be determined such that the total amount, including the amount of Rh, is within the aforementioned range. The amount of Pt supported in the second powder is preferably within the range of 0.1-10 g relative to 120 g of the porous particles. If the amount of Pt supported is less than 0.1 g relative to 120 g of the porous particles, the removing rates for HC, CO and NOx decrease. If it is more than 10 g relative to 120 g, no further improvement of the effect is achieved and a cost increase may result. It is also possible to support Pd in the second powder, in addition to Pt. Preferably, the amount of Pd is determined such that the total amount of Pt and Pd is within the range of 0.1-10 g relative to 120 g of the porous particles. The NOx adsorbent may be formed of at least one element selected from alkaline metals, alkaline earth metals and rare earth metals. Examples of the alkali metals include lithium (Li), sodium (Na), potassium (K), cesium (Cs), and the like. The alkaline earth metals refer to elements of the 2A family according to the periodic table. Examples of the alkaline earth metals include magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and the like. Examples of the rare earth metals include lanthanum (La), cerium (Ce), praseodymium (Pr), and the like. The amount of the NOx adsorbent supported in the second powder is preferably within the range of 0.05-3 moles relative to 120 g of the porous particles. If the amount of the NOx adsorbent in the second powder is less than 0.05 mole relative to 120 g of the porous particles, the NOx removing rate decreases. If the amount is more than 3 moles relative to 120 g of the porous particles, no further improvement is achieved. A particularly preferred range of the amount of the NOx adsorbent in the second powder is 0.05-0.5 mole relative to 120 g of the porous particles. The mixing ratio between the first powder and the second powder is preferably within the range of first powder : second powder = 0.05:1 to 1:1, on the basis of weight ratio between Rh and Pt or the total weight of Pt and Pd. If alumina is used as the porous particles of both the first powder and the second powder, the mixing ratio is preferably within the range of first powder : second powder = 0.1:1 to 2:1, on an alumina weight ratio basis. If the mixing ratio is outside these ranges, there may occur problems similar to those caused by inappropriate amounts of Rh and Pt. To form an exhaust gas purifying catalyst from a mixture of the first powder and the second powder, the mixture may be formed into pellets by a standard method, thereby producing a catalyst in the form of pellets. Furthermore, a slurry containing a mixture of the first and second powders as a main component may be applied as a coating onto a cordierite or metal foil-formed honeycomb base, which is then baked to produce a monolithic catalyst. In the exhaust gas purifying method of the invention, HC and CO are removed by oxidation on Pt in the second powder in a lean atmosphere containing an excessive amount of oxygen. Simultaneously, NO is oxidized into NOx, which is rapidly adsorbed by the NOx adsorbent supported adjacent to Pt. Since Pt is supported apart from Rh, inhibition of the oxidizing performance of Pt by Rh is prevented and, therefore, NO is smoothly converted into NOx. Furthermore, since the Nox adsorbent is supported apart from Rh, the deterioration of the NOx removing performance is prevented. Therefore, NOx is smoothly adsorbed by the NOx adsorbent and thereby prevented from being released to outside. HC and CO in exhaust gas react with oxygen present in an excessive amount by catalytic reactions of Pt and Rh. HC and CO are thus readily removed by oxidation. When the atmosphere is temporarily changed to a stoichiometric-rich atmosphere, NOx is released from the NOx adsorbent, and reacts with HC and CO in the exhaust gas by the catalytic reactions of Pt and Rh and, in addition, reacts with hydrogen produced through the reactions of formulas (1) and (2), which are accelerated by at least one element selected from Co, Fe and Ni. NOx is thereby reduced into N 2 . Even if Pt particle growth occurs so that the reducing power decreases, the great reducing power of Rh still efficiently removes NOx by reduction. EXAMPLES The invention will be further described in detail with reference to Examples and Comparative Examples, and the accompanying drawings. EXAMPLE 1 Figs. 1-3 illustrate an exhaust gas purifying catalyst of Example 1. The catalyst of this example is formed of a cordierite-formed honeycomb base 101, and coating layers 102 formed on surfaces of the honeycomb base 101. The coating layers 102 are formed of alumina and zirconia. In the coating layers 102, the first powder 23 formed of zirconia and the second powder 24 formed of alumina are present in a mixed state as shown in Figs. 2 and 3. The second powder 24 supports Ba. In an interior portion of a coating layer 23 as shown in Fig. 2, particles of the first powder 23 only support Rh 25, and Pt 26 is not supported on the particles of the first powder 23. Particles of the second powder 24 supports Pt 26 and Rh 27. In a surface portion 103 of a coating layer 102 as shown in Fig. 3, the structure is similar to the interior structure shown in Fig. 2, but a larger amount of Pt is supported on most particles of the first and second powders 23, 24. That is, the concentration of Pt supported is higher in the surface portion 103 than in the interior portion. The method of producing the exhaust gas purifying catalyst will be described below. The structure of the exhaust gas purifying catalyst will. <Preparation of First Powder> 480 g of zirconia powder was dipped into a rhodium nitrate aqueous solution of a predetermined concentration. After being filtered, the powder material was dried at 110 °C for 3 hours, and ground, and then baked at 400 °C for 1 hour, thereby supporting Rh in the powder. The amount of Rh supported was 2.0 g relative to 480 g of the zirconia powder. The first powder was thus prepared. <Preparation of Second Powder> 480 g of γ-alumina powder was impregnated with a predetermined amount of a barium acetate aqueous solution of a predetermined concentration. The powder material was then dried at 110 °C for 3 hours while being stirred, thereby evaporating moisture. After being ground, the powder was baked at 500 °C for 3 hours, thereby supporting Ba in the powder. The amount of Ba supported was 1.6 moles relative to 480 g of the alumina powder. Subsequently, the thus-obtained Ba-carrying alumina powder was dipped into 6 L of a 0.3-mol/L ammonium hydrogen carbonate aqueous solution. After being stirred for 15 minutes, the mixture was filtered, and dried at 110 °C for 3 hours, and then ground. Thereby, Ba was converted into barium carbonate and supported uniformly in the alumina powder. The Ba/alumina powder was dipped into a dinitrodiammine platinum nitrate aqueous solution of a predetermined concentration. After being filtered, the powder material was dried at 110 °C for 3 hours, and then ground. Subsequently, the powder was dried at 400 °C for 2 hours, thereby supporting Pt in the powder. The amount of Pt supported was 6.4 g relative to 480 g of the alumina powder. Subsequently, the obtained powder was dipped into a rhodium nitrate aqueous solution of a predetermined concentration. After being filtered, the powder material was dried at 110 °C for 3 hours, and then ground. Subsequently, the powder was dried at 400 °C for 2 hours, thereby supporting Rh in the powder. The amount of Rh supported was 0.32 g relative to 480 g of the alumina powder. The second powder was thus prepared. <Formation of Coating Layer> The entire amounts of the first powder and the second powder were uniformly mixed, and turned into a slurry by a standard method. A cordierite-formed honeycomb monolithic base member having a capacity of 1.3 L was dipped into the slurry and then pulled up. After an extraneous amount of the slurry was blown off, the monolithic base member was dried and baked, thereby forming coating layers. The amount of coating layers formed was 321.1 g relative to 1 L of the honeycomb base. The coating layers contained 120 g of zirconia as the first powder and 120 g of alumina as the second powder. <Supporting of Pt and Rh in Surface Portion> The thus-coated honeycomb base was dipped into a dinitrodiammine platinum nitrate aqueous solution of a predetermined concentration, and then pulled up. After an extraneous amount of the solution was blown off, the honeycomb base was dried at 110 °C for 3 hours. The honeycomb base was then dipped into a rhodium nitrate aqueous solution of a predetermined concentration, and pulled up. After an extraneous amount of the solution was blown off, the honeycomb base was dried at 110 °C for 3 hours, and then baked at 400 °C for 1 hour, thereby obtaining a catalyst of Example 1. Pt was supported in an amount of 0.52 g (0.4 g relative to 1 L of the honeycomb base). Rh was supported in an amount of 0.026 g (0.02 g relative to 1 L of the honeycomb base). Both elements were supported in a surface portion of the coating layer. EXAMPLE 2 <Preparation of First Powder> 480 g of zirconia powder was dipped into a rhodium nitrate aqueous solution of a predetermined concentration. After being filtered, the powder material was dried at 110 °C for 3 hours, and ground, and then baked at 400 °C for 1 hour, thereby supporting Rh in the powder. The amount of Rh supported was 2.0 g relative to 480 g of the zirconia powder. The first powder was thus prepared. <Preparation of second Powder> 480 g of γ-alumina powder was impregnated with a predetermined amount of a barium acetate aqueous solution of a predetermined concentration. The powder material was then dried at 110°C for 3 hours while being stirred, thereby evaporating moisture. After being ground, the powder was baked at 500°C for 3 hours, thereby supporting Ba in the powder. The amount of Ba supported was 1. 6 moles relative to 480 g of the alumina powder. Subsequently, the thus-obtained Ba-carrying alumina powder was dipped into 6 L of a 0.3-mol/L ammonium hydrogen carbonate aqueous solution. After being stirred for 15 minutes, the mixture was filtered, and dried at 110 °C for 3 hours, and then ground. Thereby, Ba was converted into barium carbonate and supported uniformly in the alumina powder. The Ba/alumina powder was dipped into a dinitrodiammine platinum nitrate aqueous solution of a predetermined concentration. After being filtered, the powder material was dried at 110°C for 3 hours, and then ground. Subsequently, the powder was dried at 400 °C for 2 hours, thereby supporting Pt in the powder. The amount of Pt supported was 4.0 g relative to 480 g of the alumina powder. Subsequently, the obtained powder was dipped into a rhodium nitrate aqueous solution of a predetermined concentration. After being filtered, the powder material was dried at 110°C for 3 hours, and then ground. Subsequently, the powder was dried at 400°C for 2 hours, thereby supporting Rh in the powder. The amount of Rh supported was 0.2 g relative to 480 g of the alumina powder. The second powder was thus prepared. <Formation of Coating Layer> The entire amounts of the first powder and the second powder were uniformly mixed, and turned into a slurry by a standard method. A cordierite-formed honeycomb monolithic base member having a capacity of 1.3 L was dipped into the slurry and then pulled up. After an extraneous amount of the slurry was blown off, the monolithic base member was dried and baked, thereby forming coating layers. The amount of coating layers formed was 320.5 g relative to 1 L of the honeycomb base. The coating layers contained 120 g of zirconia as the first powder and 120 g of alumina as the second powder. <Supporting of Pt and Rh in Surface Portion> The thus-coated honeycomb base was dipped into a dinitrodiammine platinum nitrate aqueous solution of a predetermined concentration, and then pulled up. After an extraneous amount of the solution was blown off, the honeycomb base was dried at 110°C for 3 hours. The honeycomb base was then dipped into a rhodium nitrate aqueous solution of a predetermined concentration, and pulled up. After an extraneous amount of the solution was blown off, the honeycomb base was dried at 110 °C for 3 hours, and then baked at 400 °C for 1 hour, thereby obtaining a catalyst of Example 2. Pt was supported in an amount of 1.3 g (1.0 g relative to 1 L of the honeycomb base). Rh was supported in an amount of 0.065 g (0.05 g relative to 1 L of the honeycomb base). Both elements were supported in a surface portion of the coating layer. EXAMPLE 3 <Preparation of First Powder> 480 g of zirconia powder was dipped into a rhodium nitrate aqueous solution of a predetermined concentration. After being filtered, the powder material was dried at 110 °C for 3 hours, and ground, and then baked at 400 °C for 1 hour, thereby supporting Rh in the powder. The amount of Rh supported was 2.0 g relative to 480 g of the zirconia powder. The first powder was thus prepared. <Preparation of Second Powder> 480 g of γ-alumina powder was impregnated with a predetermined amount of a barium acetate aqueous solution of a predetermined concentration. The powder material was then dried at 110 °C for 3 hours while being stirred, thereby evaporating moisture. After being ground, the powder was baked at 500 °C for 3 hours, thereby supporting Ba in the powder. The amount of Ba supported was 1.6 moles relative to 480 g of the alumina powder. Subsequently, the thus-obtained Ba-carrying alumina powder was dipped into 6 L of a 0.3-mol/L ammonium hydrogen carbonate aqueous solution. After being stirred for 15 minutes, the mixture was filtered, and dried at 110 °C for 3 hours, and then ground. Thereby, Ba was converted into barium carbonate and supported uniformly in the alumina powder. The Ba/alumina powder was dipped into a dinitrodiammine platinum nitrate aqueous solution of a predetermined concentration. After being filtered, the powder material was dried at 110 °C for 3 hours, and then ground. Subsequently, the powder was dried at 400 °C for 2 hours, thereby supporting Pt in the powder. The amount of Pt supported was 1.6 g relative to 480 g of the alumina powder. Subsequently, the obtained powder was dipped into a rhodium nitrate aqueous solution of a predetermined concentration. After being filtered, the powder material was dried at 110 °C for 3 hours, and then ground. Subsequently, the powder was dried at 400 °C for 2 hours, thereby supporting Rh in the powder. The amount of Rh supported was 0.08 g relative to 480 g of the alumina powder. The second powder was thus prepared. <Formation of Coating Layer> The entire amounts of the first powder and the second powder were uniformly mixed, and turned into a slurry by the standard method. A cordierite-formed honeycomb monolithic base member having a capacity of 1.3 L was dipped into the slurry and then pulled up. After an extraneous amount of the slurry was blown off, the monolithic base member was dried and baked, thereby forming coating layers. The amount of coating layers formed was 319.7 g relative to 1 L of the honeycomb base. The coating layers contained 120 g of zirconia as the first powder and 120 g of alumina as the second powder. <Supporting of Pt and Rh in Surface Portion> The thus-coated honeycomb base was dipped into a dinitrodiammine platinum nitrate aqueous solution of a predetermined concentration, and then pulled up. After an extraneous amount of the solution was blown off, the honeycomb base was dried at 110 °C for 3 hours. The honeycomb base was then dipped into a rhodium'nitrate aqueous solution of a predetermined concentration, and pulled up. After an extraneous amount of the solution was blown off, the honeycomb base was dried at 110 °C for 3 hours, and then baked at 400 °C for 1 hour, thereby obtaining a catalyst of Example 3. Pt was supported in an amount of 2.08 g (1.6 g relative to 1 L of the honeycomb base). Rh was supported in an amount of 0.104 g (0.08 g relative to 1 L of the honeycomb base). Both elements were supported in a surface portion of the coating layer. EXAMPLE 4 <Preparation of First Powder> 480 g of zirconia powder was dipped into a rhodium nitrate aqueous solution of a predetermined concentration. After being filtered, the powder material was dried at 110 °C for 3 hours, and ground, and then baked at 400 °C for 1 hour, thereby supporting Rh in the powder. The amount of Rh supported was 2.0 g relative to 480 g of the zirconia powder. The first powder was thus prepared. <Preparation of Second Powder> 480 g of γ-alumina powder was impregnated with a predetermined amount of a barium acetate aqueous solution of a predetermined concentration. The powder material was then dried at 110 °C for 3 hours while being stirred, thereby evaporating moisture. After being ground, the powder was baked at 500 °C for 3 hours, thereby supporting Ba in the powder. The amount of Ba supported was 1.6 moles relative to 480 g of the alumina powder. Subsequently, the thus-obtained Ba-carrying alumina powder was dipped into 6 L of a 0.3-mol/L ammonium hydrogen carbonate aqueous solution. After being stirred for 15 minutes, the mixture was filtered, and dried at 110 °C for 3 hours, and then ground. Thereby, Ba was converted into barium carbonate and supported uniformly in the alumina powder. <Formation of Coating Layer> The entire amounts of the first powder and the second powder were uniformly mixed, and turned into a slurry by the standard method. A cordierite-formed honeycomb monolithic base member having a capacity of 1.3 L was dipped into the slurry and then pulled up. After an extraneous amount of the slurry was blown off, the monolithic base member was dried and baked, thereby forming coating layers. The amount of coating layers formed was 319.3 g relative to 1 L of the honeycomb base. The coating layers contained 120 g of zirconia as the first powder and 120 g of alumina as the second powder. <Supporting of Pt and Rh in Surface Portion> The thus-coated honeycomb base was dipped into a dinitrodiammine platinum nitrate aqueous solution of a predetermined concentration, and then pulled up. After an extraneous amount of the solution was blown off, the honeycomb base was dried at 110 °C for 3 hours. The honeycomb base was then dipped into a rhodium nitrate aqueous solution of a predetermined concentration, and pulled up. After an extraneous amount of the solution was blown off, the honeycomb base was dried at 110 °C for 3 hours, and then baked at 400 °C for 1 hour, thereby obtaining a catalyst of Example 4. Pt was supported in an amount of 2.6 g (2.0 g relative to 1 L of the honeycomb base). Rh was supported in an amount of 0.13 g (0.1 g relative to 1 L of the honeycomb base). Both elements were supported in a surface portion of the coating layer. EXAMPLE 5 <Preparation of First Powder> 480 g of zirconia powder was dipped into a rhodium nitrate aqueous solution of a predetermined concentration. After being filtered, the powder material was dried at 110 °C for 3 hours, and ground, and then baked at 400 °C for 1 hour, thereby supporting Rh in the powder. The amount of Rh supported was 2.0 g relative to 480 g of the zirconia powder. The first powder was thus prepared. <Preparation of Second Powder> A mixture of 480 g of γ-alumina powder and 120 g of titania powder was impregnated with a predetermined amount of a barium acetate aqueous solution of a predetermined concentration. The powder material was then dried at 110 °C for 3 hours while being stirred, thereby evaporating moisture. After being ground, the powder was baked at 500 °C for 3 hours, thereby supporting Ba in the powder. The amount of Ba supported was 1.6 moles relative to 600 g of the powder combining 480 g of the alumina powder and 120 g of the titania. The thus-obtained powder was dipped into 6 L of a 0.3-mol/L ammonium hydrogen carbonate aqueous solution. After being stirred for 15 minutes, the mixture was filtered, and dried at 110 °C for 3 hours, and then ground. Thereby, Ba was converted into barium carbonate and supported uniformly in the alumina powder and the titania. The Ba-carrying alumina-titania powder was dipped into a dinitrodiammine platinum nitrate aqueous solution of a predetermined concentration. After being filtered, the powder material was dried at 110 °C for 3 hours, and then ground. Subsequently, the powder was dried at 400 °C for 2 hours, thereby supporting Pt in the powder. The amount of Pt supported was 4.0 g relative to 600 g of the powder combining 480 g of the alumina powder and 120 of the titania powder. Subsequently, the obtained powder was dipped into a rhodium nitrate aqueous solution of a predetermined concentration. After being filtered, the powder material was dried at 110 °C for 3 hours, and then ground. Subsequently, the powder was dried at 400°C for 2 hours, thereby supporting Rh in the powder. The amount of Rh supported was 0.2 g relative to 600 g of the powder combining 480 g of the alumina powder. The second powder was thus prepared. <Formation of Coating Layer> The entire amounts of the first powder and the second powder were uniformly mixed, and turned into a slurry by the standard method. A cordierite-formed honeycomb monolithic base member having a capacity of 1.3 L was dipped into the slurry and then pulled up. After an extraneous amount of the slurry was blown off, the monolithic base member was dried and baked, thereby forming coating layers. The amount of coating layers formed was 320.5 g relative to 1 L of the honeycomb base. The coating layers contained 96 g of zirconia as the first powder and 144 g in total of alumina and titania as the second powder. <Supporting of Pt and Rh in Surface Portion> The thus-coated honeycomb base was dipped into a dinitrodiammine platinum nitrate aqueous solution of a predetermined concentration, and then pulled up. After an extraneous amount of the solution was blown off, the honeycomb base was dried at 110 °C for 3 hours. The honeycomb base was then dipped into a rhodium nitrate aqueous solution of a predetermined concentration, and pulled up. After an extraneous amount of the solution was blown off, the honeycomb base was dried at 110 °C for 3 hours, and then baked at 400 °C for 1 hour, thereby obtaining a catalyst of Example 5. Pt was supported in an amount of 1.3 g (1.0 g relative to 1 L of the honeycomb base). Rh was supported in an amount of 0.065 g (0.05 g relative to 1 L of the honeycomb base). Both elements were supported in a surface portion of the coating layer. EXAMPLE 6 <Preparation of First Powder> 480 g of zirconia powder was dipped into a rhodium nitrate aqueous solution of a predetermined concentration. After being filtered, the powder material was dried at 110 °C for 3 hours, and ground, and then baked at 400 °C for 1 hour, thereby supporting Rh in the powder. The amount of Rh supported was 2.0 g relative to 480 g of the zirconia powder. The first powder was thus prepared. <Preparation of Second Powder> A mixture of 480 g of γ-alumina powder and 120 g of titania powder was impregnated with a predetermined amount of a barium acetate aqueous solution of a predetermined concentration. The powder material was then dried at 110 °C for 3 hours while being stirred, thereby evaporating moisture. After being ground, the powder was baked at 500 °C for 3 hours, thereby supporting Ba in the powder. The amount of Ba supported was 1.6 moles relative to 600 g of the powder combining 480 g of the alumina powder and 120 g of the titania. The thus-obtained powder was dipped into 6 L of a 0.3-mol/L ammonium hydrogen carbonate aqueous solution. After being stirred for 15 minutes, the mixture was filtered, and dried at 110 °C for 3 hours, and then ground. Thereby, Ba was converted into barium carbonate and supported uniformly in the alumina powder and the titania. The Ba-carrying alumina-titania powder was dipped into a dinitrodiammine platinum nitrate aqueous solution of a predetermined concentration. After being filtered, the powder material was dried at 110 °C for 3 hours, and then ground. Subsequently, the powder was dried at 400 °C for 2 hours, thereby supporting Pt in the powder. The amount of Pt supported was 4.0 g relative to 600 g of the powder combining 480 g of the alumina powder and 120 of the titania powder. Subsequently, the obtained powder was dipped into a rhodium nitrate aqueous solution of a predetermined concentration. After being filtered, the powder material was dried at 110 °C for 3 hours, and then ground. Subsequently, the powder was dried at 400 °C for 2 hours, thereby supporting Rh in the powder. The amount of Rh supported was 0.2 g relative to 600 g of the powder combining 480 g of the alumina powder and 120 of the titania powder. The second powder was thus prepared. <Formation of Coating Layer> The entire amounts of the first powder and the second powder were uniformly mixed, and turned into a slurry by the standard method. A cordierite-formed honeycomb monolithic base member having a capacity of 1.3 L was dipped into the slurry and then pulled up. After an extraneous amount of the slurry was blown off, the monolithic base member was dried and baked, thereby forming coating layers. The amount of coating layers formed was 320.5 g relative to 1 L of the honeycomb base. The coating layers contained zirconia as the first powder in an amount 96 g, and alumina and titania as the second powder in a total amount of 144g. <Supporting of Pt, Rh, K and Li in Surface Portion> The thus-coated honeycomb base was dipped into a dinitrodiammine platinum nitrate aqueous solution of a predetermined concentration, and then pulled up. After an extraneous amount of the solution was blown off, the honeycomb base was dried at 110 °C for 3 hours. The honeycomb base was then dipped into a rhodium nitrate aqueous solution of a predetermined concentration, and pulled up. After an extraneous amount of the solution was blown off, the honeycomb base was dried at 110 °C for 3 hours, and then baked at 400 °C for 1 hour. Subsequently, the honeycomb base was dipped into a potassium acetate aqueous solution of a predetermined amount, and pulled up. After an extraneous amount of the solution was blown off, the honeycomb base was dried at 110 °C for 3 hours. The honeycomb base was then dipped into a lithium nitrate aqueous solution of a predetermined concentration, and pulled up. After an extraneous amount of the solution was blown off, the honeycomb base was dried at 110 °C for 3 hours, and then baked at 500 °C for 1 hour, thereby obtaining a catalyst of Example 6. Pt was supported in an amount of 1.3 g (1.0 g relative to 1 L of the honeycomb base). Rh was supported in an amount of 0.065 g (0.05 g relative to 1 L of the honeycomb base). Each of potassium (K) and lithium (Li) was supported in an amount of 0.13 mol (0.1 mole relative to 1 L of the honeycomb base). These elements were supported in a surface portion of the coating layer. EXAMPLE 7 <Preparation of First Powder> 384 g of barium-stabiLized zirconia powder (containing 1 mol% Ba) was dipped into a rhodium nitrate aqueous solution of a predetermined concentration. After being filtered, the powder material was dried at 110 °C for 3 hours, and ground, and then baked at 400 °C for 1 hour, thereby supporting Rh in the powder. The amount of Rh supported was 2.0 g relative to 384 g of the zirconia powder. The first powder was thus prepared. <Preparation of Second Powder> A mixture of 480 g of γ-alumina powder and 120 g of titania powder was impregnated with a predetermined amount of a barium acetate aqueous solution of a predetermined concentration. The powder material was then dried at 110 °C for 3 hours while being stirred, thereby evaporating moisture. After being ground, the powder was baked at 500 °C for 3 hours, thereby supporting Ba in the powder. The amount of Ba supported was 1. 6 moles relative to 600 g of the powder combining 480 g of the alumina powder and 120 g of the titania. The thus-obtained powder was dipped into 6 L of a 0.3-mol/L ammonium hydrogen carbonate aqueous solution. After being stirred for 15 minutes, the mixture was filtered, and dried at 110 °C for 3 hours, and then ground. Thereby, Ba was converted into barium carbonate and supported uniformly in the alumina powder and the titania. The Ba-carrying alumina-titania powder was dipped into a dinitrodiammine platinum nitrate aqueous solution of a predetermined concentration. After being filtered, the powder material was dried at 110 °C for 3 hours, and then ground. Subsequently, the powder was dried at 400 °C for 2 hours, thereby supporting Pt in the powder. The amount of Pt supported was 4.0 g relative to 600 g of the powder combining 480 g of the alumina powder and 120 g of the titania powder. Subsequently, the obtained powder was dipped into a rhodium nitrate aqueous solution of a predetermined concentration. After being filtered, the powder material was dried at 110 °C for 3 hours, and then ground. Subsequently, the powder was dried at 400 °C for 2 hours, thereby supporting Rh in the powder. The amount of Rh supported was 0.2 g relative to 600 g of the powder combining 480 g of the alumina powder and 120 g of the titania powder. The second powder was thus prepared. <Formation of Coating Layer> The entire amounts of the first powder and the second powder were uniformly mixed, and turned into a slurry by the standard method. A cordierite-formed honeycomb monolithic base member having a capacity of 1.3 L was dipped into the slurry and then pulled up. After an extraneous amount of the slurry was blown off, the monolithic base member was dried and baked, thereby forming coating layers. The amount of coating layers formed was 320.5 g relative to 1 L of the honeycomb base. The coating layers contained zirconia as the first powder in an amount 96 g, and alumina and titania as the secorid powder in a total amount of 144g. <Supporting of Pt, Rh, K and Li in Surface Portion> The thus-coated honeycomb base was dipped into a dinitrodiammine platinum nitrate aqueous solution of a predetermined concentration, and then pulled up. After an extraneous amount of the solution was blown off, the honeycomb base was dried at 110 °C for 3 hours. The honeycomb base was then dipped into a rhodium nitrate aqueous solution of a predetermined concentration, and pulled up. After an extraneous amount of the solution was blown off, the honeycomb base was dried at 110 °C for 3 hours, and then baked at 400 °C for 1 hour. Subsequently, the honeycomb base was dipped into a potassium acetate aqueous solution of a predetermined amount, and pulled up. After an extraneous amount of the solution was blown off, the honeycomb base was dried at 110 °C or 3 hours. The honeycomb base was then dipped into a lithium nitrate aqueous solution of a predetermined concentration, and pulled up. After an extraneous amount of the solution was blown off, the honeycomb base was dried at 110 °C for 3 hours, and then baked at 500 °C for 1 hour, thereby obtaining a catalyst of Example 7. Pt was supported in an amount of 1.3 g (1.0 g relative to 1 L of the honeycomb base). Rh was supported in an amount of 0.065 g (0.05 g relative to 1 L of the honeycomb base). Each of K and Li was supported in an amount of 0.13 mol (0.1 mole relative to 1 L of the honeycomb base). These elements were supported in a surface portion of the coating layer. COMPARATIVE EXAMPLE 1 γ-Alumina was turned into a slurry by the standard method. A cordierite-formed honeycomb monolithic base member having a capacity of 1.3 L was dipped into the slurry and then pulled up. After an extraneous amount of the slurry was blown off, the monolithic base member was dried and baked, thereby forming coating layers. The amount of coating layers formed was 240.0 g relative to 1 L of the honeycomb base. The thus-coated honeycomb base was dipped into a barium acetate aqueous solution of a predetermined concentration, and then pulled up. After an extraneous amount of the solution was blown off, the honeycomb base was baked at 500 °C for 3 hours, thereby supporting 0.4 mole of Ba. After that, the honeycomb base was entirely dipped into 2 L of a 0.3-mol/L ammonium hydrogen carbonate aqueous solution. The solution was stirred for 15 minutes followed by filtration. The honeycomb base was then dried at 110 °C for 3 hours, and ground. Ba was thereby converted into barium carbonate and supported uniformly on surfaces of the alumina coating layers. The honeycomb base having alumina coating layers carrying barium carbonate was dipped into a dinitrodiammine platinum nitrate aqueous solution of a predetermined concentration, and then pulled up. After an extraneous amount of the solution was blown off, the honeycomb base was dried at 110 °C for 3 hours. The honeycomb base was then dipped into a rhodium nitrate aqueous solution of a predetermined concentration, and pulled up. After an extraneous amount of the solution was blown off, the honeycomb base was dried at 110 °C for 3 hours, and then baked at 400 °C for 1 hour, thereby obtaining a catalyst of Comparative Example 1. Pt was supported in an amount of 2.6 g (2.0 g relative to 1 L of the honeycomb base). Rh was supported in an amount of 0.78 g (0.6 g relative to 1 L of the honeycomb base). Both elements were supported in a surface portion of the coating layer. COMPARATIVE EXAMPLE 2 After 240 g of γ-alumina and 240 g of zirconia powder were thoroughly mixed, the mixture was turned into a slurry by the standard method. A cordierite-formed honeycomb monolithic base member having a capacity of 1.3 L was dipped into the slurry and then pulled up. After an extraneous amount of the slurry was blown off, the monolithic base member was dried and baked, thereby forming coating layers. The amount of coating layers formed was 240.0 g relative to 1 L of the honeycomb base. The thus-coated honeycomb base was dipped into a barium acetate aqueous solution of a predetermined concentration, and then pulled up. After an extraneous amount of the solution was blown off, the honeycomb base was baked at 500 °C for 3 hours, thereby supporting 0.4 mole of Ba. After that, the honeycomb base was entirely dipped into 2 L of a 0.3-mol/L ammonium hydrogen carbonate aqueous solution. The solution was stirred for 15 minutes followed by filtration. The honeycomb base was then dried at 110 °C for 3 hours, and ground. Ba was thereby converted into barium carbonate and supported uniformly on surfaces of the alumina/zirconia coating layers. The honeycomb base having alumina/zirconia coating layers carrying barium carbonate was dipped into a dinitrodiammine platinum nitrate aqueous solution of a predetermined concentration, and then pulled up. After an extraneous amount of the solution was blown off, the honeycomb base was dried at 110 °C for 3 hours. The honeycomb base was then dipped into a rhodium nitrate aqueous solution of a predetermined concentration, and pulled up. After an extraneous amount of the solution was blown off, the honeycomb base was dried at 110 °C for 3 hours, and then baked at 400 °C for 1 hour, thereby obtaining a catalyst of Comparative Example 2. Pt was supported in an amount of 2.6 g (2.0 g relative to 1 L of the honeycomb base). Rh was supported in an amount of 0.78 g (0.6 g relative to 1 L of the honeycomb base). Both elements were supported in a surface portion of the coating layer. COMPARATIVE EXAMPLE 3 After 240 g of γ-alumina, 192 g of zirconia powder and 48 g of titania powder were thoroughly mixed, the mixture was turried into a slurry by the standard method. A cordierite-formed honeycomb monolithic base member having a capacity of 1.3 L was dipped into the slurry and then pulled up. After an extraneous amount of the slurry was blown off, the monolithic base member was dried and baked, thereby forming coating layers. The amount of coating layers formed was 240.0 g relative to 1 L of the honeycomb base. The thus-coated honeycomb base was dipped into a barium acetate aqueous solution of a predetermined concentration, and then pulled up. After an extraneous amount of the solution was blown off, the honeycomb base was baked at 500 °C for 3 hours, thereby supporting 0.4 mole of Ba. After that, the honeycomb base was entirely dipped into 2 L of a 0.3-mol/L ammonium hydrogen carbonate aqueous solution. The solution was stirred for 15 minutes followed by filtration. The honeycomb base was then dried at 110 °C for 3 hours, and ground. Ba was thereby converted into barium carbonate and supported uniformly on surfaces of the alumina/zirconia/titania coating layers. The honeycomb base having alumina/zirconia/titania coating layers carrying barium carbonate was dipped into a dinitrodiammine platinum nitrate aqueous solution of a predetermined concentration, and then pulled up. After an extraneous amount of the solution was blown off, the honeycomb base was dried at 110 °C for 3 hours. The honeycomb base was then dipped into a rhodium nitrate aqueous solution of a predetermined concentration, and pulled up. After an extraneous amount of the solution was blown off, the honeycomb base was dried at 110 °C for 3 hours, and then baked at 400 °C for 1 hour, thereby obtaining a catalyst of Comparative Example 3. Pt was supported in an amount of 2.6 g (2.0 g relative to 1 L of the honeycomb base). Rh was supported in an amount of 0.78 g (0.6 g relative to 1 L of the honeycomb base). Both elements were supported in a surface portion of the coating layer. COMPARATIVE EXAMPLE 4 After 240 g of γ-alumina, 192 g of zirconia powder and 48 g of titariia powder were thoroughly mixed, the mixture was turned into a slurry by the standard method. A cordierite-formed honeycomb monolithic base member having a capacity of 1.3 L was dipped into the slurry and then pulled up. After an extraneous amount of the slurry was blown off, the monolithic base member was dried and baked, thereby forming coating layers. The amount of coating layers formed was 240.0 g relative to 1 L of the honeycomb base. The thus-coated honeycomb base was dipped into a barium acetate aqueous solution of a predetermined concentration, and then pulled up. After an extraneous amount of the solution was blown off, the honeycomb base was baked at 500 °C for 3 hours, thereby supporting 0.4 mole of Ba. After that, the honeycomb base was entirely dipped into 2 L of a 0.3-mol/L ammonium hydrogen carbonate aqueous solution. The solution was stirred for 15 minutes followed by filtration. The honeycomb base was then dried at 110 °C for 3 hours, and ground. Ba was thereby converted into barium carbonate and supported uniformly on surfaces of the alumina/zirconia/titania coating layers. The honeycomb base having alumina/zirconia/titania coating layers carrying barium carbonate was dipped into a dinitrodiammine platinum nitrate aqueous solution of a predetermined concentration, and then pulled up. After an extraneous amount of the solution was blown off, the honeycomb base was dried at 110 °C for 3 hours. The' honeycomb base was then dipped into a rhodium nitrate aqueous solution of a predetermined concentration, and pulled up. After an extraneous amount of the solution was blown off, the honeycomb base was dried at 110 °C for 3 hours, and then baked at 400 °C for 1 hour. Pt was supported in an amount of 2.6 g (2.0 g relative to 1 L of the honeycomb base). Rh was supported in an amount of 0.78 g (0.6 g relative to 1 L of the honeycomb base). Both elements were supported in a surface portion of the coating layer. Subsequently, the honeycomb base was dipped into a potassium acetate aqueous solution of a predetermined amount, and pulled up. After an extraneous amount of the solution was blown off, the honeycomb base was dried at 110 °C for 3 hours. The honeycomb base was then dipped into a lithium nitrate aqueous solution of a predetermined concentration, and pulled up. After an extraneous amount of the solution was blown off, the honeycomb base was dried at 110°C for 3 hours, and then baked at 500°C for 1 hour, thereby obtaining a catalyst of Comparative Example 4. Each of potassium (K) and lithium (Li) was supported in an amount of 0.13 mol (0.1 mole relative to 1 L of the honeycomb base). COMPARATIVE EXAMPLE 5 <Preparation of First Powder> 480 g of zirconia powder was dipped into a rhodium nitrate aqueous solution of a predetermined concentration. After being filtered, the powder material was dried at 110°C for 3 hours, and ground, and then baked at 400 °C for 1 hour, thereby supporting Rh in the powder. The amount of Rh supported was 2.0 g relative to 480 g of the zirconia powder. The first powder was thus prepared. <Preparation of Second Powder> 480 g of γ-alumina powder was impregnated with a predetermined amount of a barium acetate aqueous solution of a predetermined concentration. The powder material was then dried at 110 °C for 3 hours while being stirred, thereby evaporating moisture. After being ground, the powder was baked at 500°C for 3 hours, thereby supporting Ba in the powder. The amount of Ba supported was 1.6 moles relative to 480 g of the alumina powder. Subsequently, the thus-obtained powder was dipped into 6 L of a 0.3-mol/L ammonium hydrogen carbonate aqueous solution. After being stirred for 15 minutes, the mixture was filtered, and dried at 110 °C for 3 hours, and then ground. Thereby, Ba was converted into barium carbonate and supported uniformly in the alumina powder. The Ba/alumina powder was then dipped into a dinitrodiammine platinum nitrate aqueous solution of a predetermined concentration. After being filtered, the powder material was dried at 110 °C for 3 hours, and then ground. Subsequently, the powder was dried at 400 °C for 2 hours, thereby supporting Pt in the powder. The amount of Pt supported was 8.0 g relative to 480 g of the alumina powder. Subsequently, the obtained powder was dipped into a rhodium nitrate aqueous solution of a predetermined concentration. After being filtered, the powder material was dried at 110 °C for 3 hours, and then ground. Subsequently, the powder was dried at 400 °C for 2 hours, thereby supporting Rh in the powder. The amount of Rh supported was 0.4 g relative to 480 g of the alumina powder. The second powder was thus prepared. <Formation of Coating Layer> The entire amounts of the first powder and the second powder were uniformly mixed, and turned into a slurry by the standard method. A cordierite-formed honeycomb monolithic base member having a capacity of 1.3 L was dipped into the slurry and then pulled up. After an extraneous amount of the slurry was blown off, the monolithic base member was dried and baked, thereby forming coating layers. The amount of coating layers formed was 321.1 g relative to 1 L of the honeycomb base. The coating layers contained 120 g of zirconia as the first powder and 120 g of alumina as the second powder. <Evaluation Test> An endurance test equivalent to 50,000 km driving was performed on the exhaust gas purifying catalysts of Examples 1-7 and Comparative Examples 1-5, by setting each catalyst in the exhaust system of a 1.8-L lean burn gasoline engine. After the test, the amounts of NOx adsorbed at an in-coming gas temperature of 400 °C during lean burn operation, the amounts of NOx reduced at an in-coming gas temperature of 400 °C during rich pulses, 50%-HC removal temperatures, and NOx emissions during 10-15 mode operation were measured in the same exhaust system. Results are shown in Table 1. As can be seen from Table 1, the catalyst of Comparative Example 5 exhibited a larger amount of NOx adsorbed and a larger amount of NOx reduced than the catalysts of Comparative Examples 1-4. The catalyst of Comparative Example 5 was also excellent in the NOx removing performance at the time of rich pulses even after the endurance. Therefore, it should be clear that by supporting Rh and Ba apart from each other as in Comparative Example 5, the NOx removing performance significantly improves. The catalysts of Examples 1-7 of the invention achieved even higher NOx removing performance than the catalyst of Comparative Example 5. This is apparently because a portion of Pt was supported in a surface portion of the coating layers in each of Examples 1-7. As understood from the above description, the exhaust gas purifying catalyst of the invention achieves high NOx removing performance both during an initial period and after the endurance test and, therefore, has high durability. If the exhaust gas purifying method of the invention is employed, production of NOx by oxidation of NO, adsorption of NOx to the NOx adsorbent, and reduction of NOx released from the NOx adsorbent smoothly progress. Therefore, it becomes possible to reliably maintain high NOx removing performance not only during an initial period but also after the endurance test. Furthermore, if at least one element selected from Co, Fe and Ni is optionally supported, the sulfur-poisoning of the NOx adsorbent is further reduced, so that durability is further enhanced. Further, as the catalyst is formed so that the concentration of Pt supported is higher in a surface portion than in an interior portion, the NOx removing performance is further improved. While the present invention has been described with reference to what are presently considered to be preferred embodiments thereof, it is to be understood that the invention is not limited to the disclosed embodiments, examples or constructions, and it is defined by the appended claims.

Claims: An exhaust gas purifying catalyst comprising: a base member; a coating layer formed on a surface of the base member, the coating layer being formed of a first powder formed of porous particles supporting rhodium and a second powder formed of porous particles supporting a nitrogen oxides-adsorbing material, the first powder and the second powder being present in a mixed state; and platinum supported in the coating layer, the concentration of platinum supported in a surface layer portion of the coating layer being higher than the concentration of platinum supported in an internal portion of the coating layer. An exhaust gas purifying catalyst according to claim 1, wherein the porous particles, whether for the first powder or the second powder, may be selected from one or more of alumina, silica, titania, zirconia, silica-alumina and zeolite, and wherein if more than one of these materials are selected, they may be used in a mixture or a composite form. An exhaust gas purifying catalyst according to claim 2, wherein zirconia is used for the first powder. An exhaust gas purifying catalyst according to claim 3, wherein alumina is used for the second powder. An exhaust gas purifying catalyst according to any of claims 1 to 4, wherein at least one of the first powder and the second powder supports at least one element selected from the group consisting of cobalt, iron and nickel. An exhaust gas purifying catalyst according to any of claims 1 to 5, wherein the second powder supports platinum and 1-10% by weight of rhodium relative to the amount of platinum. An exhaust gas purifying catalyst according to any of claims 1 to 6, wherein a hydrocarbon-adsorbing adsorbent is provided adjacent to the first powder. An exhaust gas purifying catalyst according to claim 7, wherein the porous particles of the first powder serve as the HC-adsorbing adsorbent. An exhaust gas purifying catalyst according to claim 7, wherein the HC-adsorbing adsorbent exists at an interface between the first powder and the second powder. An exhaust gas purifying method comprising the steps of: disposing, in an exhaust gas passage, a catalyst as claimed in any of claims 1 to 9; allowing the nitrogen oxides-adsorbing material to adsorb nitrogen oxides in a lean atmosphere in which an excessive amount of oxygen is present; and temporarily changing the lean atmosphere to a stoichiometric-rich atmosphere so as to cause reduction of nitrogen oxides released from the nitrogen oxides-adsorbing material.

Inventors: HIRATA HIROHITO, IKEDA YASUO, HIRATA, HIROHITO, IKEDA, YASUO, Hirata, Hirohito, c/o Toyota Jidosha K. K., Ikeda, Yasuo, c/o Toyota Jidosha K. K.
Applicants: TOYOTA MOTOR CO LTD, TOYOTA JIDOSHA KABUSHIKI KAISHA
Application year: 1997
Main classification: B01J23
IPC codes: B01J37, B01J23, B01J35, F01N3, B01J29, B01D53