PROCESS FOR PRODUCING EXHAUST GAS PURIFYING CATALYST, AND EXHAUST GAS PURIFYING CATALYST

Technical Field The present invention relates to an exhaust gas purifying catalyst for purifying the components in an exhaust gas, discharged from a combustion apparatus such as internal combustion engine, and to a process for producing the exhaust gas purifying catalyst. Related Art The exhaust gas from internal combustion engines such as automobile engine contains nitrogen oxide (NOx) , carbon monoxide (CO), hydrocarbon (HC) and the like. These substances can be purified by an exhaust gas purifying catalyst of oxidizing CO and HC and, at the same time, reducing NOx. As for representative exhaust gas purifying catalysts, three-way catalysts wherein a noble metal such as platinum (Pt), rhodium (Rh) and palladium (Pd) is supported on a porous metal oxide support such as γ-alumina are known. With respect to such an exhaust gas purifying catalyst, various studies has been made and a technique of mixing or stacking multiple species of metal oxide supports to utilize characteristic properties of respective metal oxide supports is also achieved. For example, ceria has an oxygen storage capacity (OSC) for storing oxygen when the oxygen concentration in the exhaust gas is high, and releasing oxygen when the oxygen concentration in the exhaust gas is low, but has a relatively low heat resistance. Accordingly, ceria is solid-dissolved or mixed with zirconia or alumina to improve heat resistance of the catalyst. Furthermore, when multiple species of metal oxide supports are mixed and used, it is also proposed to load different noble metals on respective metal oxide supports. For example, Japanese Unexamined Patent Publication (Kokai) No. 11-267503 discloses a catalyst obtained by mixing a first catalyst powder having supported thereon a noble metal and a second catalyst powder having supported thereon an NOx-storing material and a base metal. According to this technique, sintering of the noble metal can be prevented by disposing a noble metal and an NOx-storing material separately from each other and, at the same time, oxidation-reduction of NOx can be accelerated by loading a base metal and an NOx- storing material in proximity. Japanese Unexamined Patent Publication No. 10-202108 proposes to load a noble metal on a catalyst support by using an organic noble metal complex. According to this technique, a first neighbor atom to an active noble metal atom can be the same noble metal atom as the active noble metal atom. Japanese Unexamined Patent Publication No. 11-246901 proposes to produce fine metal particles in a polyhydric alcohol and prevent aggregation of fine metal particles by adjusting the pH to 2 or less or 7 or more. Japanese Unexamined Patent Publication 11-192432 proposes to use a noble metal cluster carbonyl compound in which the total electric charge n of the noble metal carbonyl complex is from -1 to -10. As described above, it is known to use multiple species of metal oxide supports, for example, ceria and alumina, in combination and thereby obtain benefits of respective supports. Also, according to studies in recent years, it has been found that the combination of a metal oxide support and a noble metal supported thereon has an important meaning. For example, when platinum is supported on ceria, sintering of platinum is prevented by virtue of affinity of platinum for ceria, and, when rhodium is supported on zirconia, a good exhaust gas purifying performance is exerted. If platinum is sintered during the use of a catalyst, active sites of the catalyst decrease and the catalytic activity is deteriorated. Therefore, it is very important to prevent sintering of platinum. As disclosed in Japanese Unexamined Patent Publication (Kokai) No. 11-267503, in order to load different noble metals on multiple species of meal oxide support powders, it is possible to previously load a noble metal on each metal oxide support powder and then mix the obtained multiple species of noble metal- supported support powders. However, this method increase the number of steps, and cannot be applied when primary particles of multiple species of metal oxide supports are aggregated to form a secondary particle, when multiple species metal oxide supports are previously stacked or mixed, or when multiple species of metal oxide supports are at least partially in solid solution. Disclosure of Invention Accordingly, the present invention provides a process for producing an exhaust gas purifying catalyst, wherein a noble metal can be selectively supported on any catalyst support out of multiple species of catalyst supports, and also provides an exhaust gas purifying catalyst obtainable by this process. The present invention is a process for producing an exhaust gas purifying catalyst, comprising the following steps (a) to (c) ; (a) providing a solution containing first and second metal oxide supports wherein the first and second metal oxide supports differ from each other in the mode of change of the zeta potential due to a change in the pH value; (b) mixing the solution with a noble metal solution containing a noble metal ion or complex ion while maintaining the pH of the solution at any one pH of the following (i) to (v) , and thereby selectively loading - A -

said noble metal on the first metal oxide support: (i) a pH where the sign of the zeta potential of the first metal oxide support is different from the sign of the zeta potential of the second metal oxide support and at the same time, different from the sign of the electric charge of the noble metal ion or complex ion; (ii) a pH where the sign of the zeta potential of the first metal oxide support is the same as the sign of the zeta potential of the second metal oxide support and different from the sign of electric charge of the noble metal ion or complex ion, and the absolute value of the zeta potential of the first metal oxide support is larger than 2 times, 3 times, 5 times or 10 times that of the zeta potential of the second metal oxide support; or (iii) a pH where the sign of the zeta potential of the first metal oxide support is the same as the sign of the zeta potential of the second metal oxide support and also the same as the sign of the electric charge of the noble metal ion or complex ion, and the absolute value of the zeta potential of the first metal oxide support is smaller than 1/2 times, 1/3 times, 1/5 times or 1/10 times that of the zeta potential of the second metal oxide support, (iv) a pH of 2 or more, particularly 3 or more, more particularly 4 or more, still more particularly 5 or more, yet still more particularly 6 of more, and 9 or less, particularly 8 or less, more particularly 7 or less, or (v) a pH where the difference between the zeta potentials of the first and second metal oxide supports exceeds 30 mV, 50 mV or 80 mV; and (c) drying and firing the obtained first and second metal oxide supports. According to this process of the present invention, the noble metal ion or complex ion can be electrostatically attracted to the first metal oxide support. Therefore, the noble metal can be more selectively supported on the first metal oxide support than on the second metal oxide support. One or multiple species of support (s) other than the first and second metal oxide supports may be further present. When the pH of (i) is used in the step (b) , the sign of the zeta potential of the first metal oxide support is different from the sign of the noble metal ion or complex ion and therefore, the noble metal ion or complex ion is electrostatically drawn to the first metal oxide support. Also, the sign of the zeta potential of the second metal oxide support is different from the sign of the electric charge of the noble metal ion or complex ion and therefore, the noble metal ion or complex ion is electrostatically repelled from the second metal oxide support. This is more outstanding when the absolute value of zeta potential of the first or second metal oxide support is large, for example, when the absolute value of zeta potential of one metal oxide support, particularly those of two metal oxide supports, exceed(s) 10 mV, 20 mV, 30 mV or 40 mV. When the pH of (ii) or (iii) is used in the step (b) , even if the sign of zeta potential of the first metal oxide support is the same as the sign of zeta potential of the second metal oxide support, the absolute values thereof differ from each other in size and, therefore, the noble metal ion or complex ion is electrostatically drawn more strongly to the first metal oxide support. This effect is more outstanding when the absolute value of zeta potential of the first or second metal oxide support is large, for example, when the absolute value of zeta potential of one metal oxide support exceeds 20 mV, 30 mV, 40 mV or 50 mV. When the pH of (iv) is used in the step (b) , the pH of the solution is adjusted to be relatively neutral, particularly, to a pH between the isoelectric point of the first metal oxide support and the isoelectric point of the second metal oxide support, thereby give a large difference between the zeta potential of the first metal oxide support and the zeta potential of the second metal oxide support. This may cause the noble metal ion or complex ion to be electrostatically drawn to the first metal oxide support. Accordingly, the noble metal can be more selectively supported on the first metal oxide support than on the second metal oxide support. When the pH of (v) is used in the step (b) , the pH of the solution is adjusted to give a large difference between zeta potentials of the first and second metal oxide supports, thereby causing the noble metal ion or complex ion to be electrostatically drawn to the metal oxide support. Accordingly, the noble metal can be more selectively supported on the first metal oxide support than on the second metal oxide support. The above- described phenomenon is more outstanding when the signs of the zeta potentials of the first and second metal oxide supports differ from each other. In one embodiment of the process of the present invention, the noble metal ion or complex ion may be a hexacoordinate noble metal complex ion, particularly a hexacoordinate platinum complex ion, more particularly a hexanitroplatinate ion (Pt (NO2) 64~) • In the hexacoordinate noble metal complex ion, the center noble metal is three-dimensionally surrounded by ligands, so that the electrostatic attractive force between the first metal oxide support and the noble metal complex ion can be more effectively utilized and the noble metal complex ions can be prevented from aggregation with each other. In another embodiment of the process of the present invention, the first and second metal oxide supports each can be independently selected from the group consisting of ceria, zirconia, alumina, titania and silica. In another embodiment of the process of the present invention, the first metal oxide support may be ceria and the noble metal solution may be a platinum solution. In this embodiment, the second metal oxide support may be zirconia or alumina. According to this embodiment, platinum can be selectively loaded on ceria by utilizing the electrostatic attractive force between ceria and platinum ion or complex ion. In another embodiment of the process of the present invention, the first metal oxide support may be zirconia and the noble metal solution may be a rhodium solution. In this embodiment, the second metal oxide support may be ceria. According to this embodiment, rhodium can be selectively loaded on zirconia by utilizing the electrostatic attractive force between zirconia and rhodium ion or complex ion. In another embodiment of the present invention, in the step (a) , the first and second metal oxide supports may be dispersed, in the form of colloid particles or a powder, in the solution. According to this embodiment wherein the first and second metal oxide supports are used in the form of a colloid particle, the first and second metal oxide fine supports originated in the colloid particles are mixed with each other in the obtained exhaust gas purifying catalyst and at the same time, the noble metal is selectively supported on the first metal oxide support. The colloid particle may have a particle diameter of, for example, 100 nm or less, 50 nm or less, 30 nm or less, or 10 nm or less. In another embodiment of the process of the present invention, in the step (a) , the first and second metal oxide supports may constitute secondary particles containing the first and second metal oxide supports. In the case of loading a noble metal on a secondary particle containing first and second metal oxide supports by a conventional method, the noble metal is equally loaded on the first and second metal oxide supports. On the other hand, according to this embodiment, the noble metal can be selectively loaded on the first metal oxide support of the secondary particle containing first and second metal oxide supports. In another embodiment of the process of the present invention, in the step (a) , the first and second metal oxide supports may be at least partially in the form of a solid solution. In the case of loading a noble metal on a secondary particle containing first and second metal oxide supports by a conventional method, the noble metal is equally loaded on the first and second metal oxide supports. On the other hand, according to this embodiment, the noble metal can be selectively loaded on the first metal oxide support moiety of the support which is at least partially in the form of a solid solution. In another embodiment of the process of the present invention, in the step (a) , the first and second metal oxides may be stacked or mixed to form a catalyst-support layer or pellets. In the case of merely loading a noble metal on a shaped catalyst-support layer or pellet by a conventional method, the noble metal is equally loaded on the first and second metal oxide supports, though the amount of the noble metal supported gradually decreases from the outer side surface toward the inner side of the catalyst- support layer or pellet. Also, in the case of previously loading a noble metal on the first metal oxide support and mixing or stacking this support with the second metal oxide support to form a catalyst-support layer or pellet, the noble metal can be selectively supported on the first metal oxide support, but the noble metal concentration becomes constant over the region from the outer side surface to the inner side of the catalyst-support layer or pellet. On the other hand, according to this embodiment, the noble metal can be selectively loaded on the first metal oxide support. Furthermore, the noble metal is loaded on a catalyst-support layer or pellet after shaping and, therefore, the noble metal is supported in a relatively high concentration on the outer side surface of the catalyst-support layer or pellet, and the concentration of the catalyst supported decreases from the outer side surface toward the inner side. The exhaust gas purifying catalyst of the present invention is an exhaust gas purifying catalyst comprising a catalyst-support layer or pellets which contain first and second metal oxide supports and on which a noble metal is supported, wherein the amount of the noble metal supported gradually decreases from the outer side surface toward the inner side of the catalyst-support layer or pellets, and the amount of the noble metal supported per unit surface area of the first metal oxide support is larger than the amount of the noble metal supported per unit area of the second metal oxide support, particularly by 50% or more, 100% or more, or 500% or more larger. The catalyst-support layer may be a layer disposed on a substrate such as honeycomb substrate. According to the exhaust gas purifying catalyst of the present invention, the amount of the noble metal supported gradually decrease from the outer side surface toward the substrate side or inner side of the catalyst- support layer or pellet. That is, the noble metal is supported in a larger amount on the outer side moiety of the catalyst-support layer or pellet, which relatively easily comes into contact with an exhaust gas, so that the noble metal supported can be effectively utilized. Furthermore, according to the exhaust gas purifying catalyst of the present invention, the amount of the noble metal supported per unit surface area of the first metal oxide support is larger than the amount of the noble metal supported per unit area of the second metal oxide support, so that the interaction between the first metal oxide support and the noble metal successfully occurs. Brief Description of the Drawings Figure 1 is a view for explaining the principle of the process of the present invention. Figure 2a is a cross-sectional view of the exhaust gas purifying catalyst layer according to the present invention. Figure 2b is a cross-sectional view of the exhaust gas purifying catalyst layer according to a conventional technique. Figure 3 is a graph showing performance of the exhaust gas purifying catalysts of Example 1 and Comparative Example 1. Figure 4 is a graph showing performance of the exhaust gas purifying catalysts of Example 2 and Comparative Example 2. Figure 5 is a graph showing performance of the exhaust gas purifying catalysts of Example 3 and Comparative Example 3. Figure 6 is a graph showing performance of the exhaust gas purifying catalysts of Examples 4 and 5. Figure 7 is a graph showing dispersibility of platinum in the exhaust gas purifying catalysts of Examples 4 and 5. Figure 8 is a graph showing performance of the exhaust gas purifying catalysts of Example 6 and Comparative Example 4. Figure 9 is a graph showing performance of the exhaust gas purifying catalysts of Example 7 and Comparative Example 5. Best Mode for Carrying Out the Invention The principle of the process of the present invention is described below, by referring to the drawings, but the present invention is not limited thereto. Fig. 1 is a view showing the change of zeta ■potentials of the metal oxide A and the metal oxide B due to the change of pH of the solution containing them. In the Figure, the curve on the left lower side shows the zeta potential of the oxide A, and the curve on the right upper side shows the zeta potential of the oxide B. As is apparent from Fig. 1, the zeta potential is changed along the change of pH in both the oxide A and the oxide B, but the mode of change differs therebetween. <pH between the isoelectric points of the oxides A and B> As shown in Fig. 1, when the pH is between about 4.7 which is the isoelectric point of the oxide A (pH at which the zeta potential or electrokinetic potential of the oxide A becomes 0), and about 7.2 which is the isoelectric point of the oxide B, the sign of the zeta potential of the oxide A is negative and the sign of the zeta potential of the oxide B is positive (C2) . The noble metal ion or complex ion contained in the noble metal solution used for loading a noble metal has a positive or negative charge. For example, tetranitroplatinate (Pt (NO2) 42~) and hexanitroplatinate (Pt (NO2) 64~) have a negative charge, whereas hexaamminerhodium (Rh(NHs)63+) has a positive charge. Accordingly, when platinum is loaded by using a tetranitroplatinate solution at a pH between about 4.7 as the isoelectric point of the oxide A and about 7.2 as the isoelectric point of the oxide B, the tetranitroplatinate having a negative charge is selectively drawn, by the Coulomb force, to the oxide B having a positive charge. Similarly, the hexanitroplatinate having a negative charge is also drawn to the oxide B having a positive charge. On the other hand, the hexaamminerhodium having a positive charge is drawn to the oxide A having a negative charge. Incidentally, according to conventional processes, a solution containing first and second metal oxide supports and a strongly acidic or strongly alkaline noble metal solution are merely mixed (for example, generally, the pH of tetranitroplatinate solution is less than 1, and the pH of hexaammineplatinum solution is from 10 to 11) . Therefore, the resulting mixed solution is strongly acidic or strongly alkaline and at, for example, a pH of less than 2 or more than 9, and, therefore, the pH of the mixed solution becomes far larger or smaller than the isoelectric points of both the first and second metal oxide supports. Accordingly, the first metal oxide support and the second metal oxide support have the same zeta potential sign and similar zeta potentials, and therefore the noble metal cannot be selectively supported. <pH smaller than the isoelectric point of the oxide A> As shown in Fig. 1, when the pH is smaller than about 4.7 which is the isoelectric point of the oxide A, both the oxide A and the oxide B have a positive zeta potential (Cl) . However, particular observation in this range reveals that, when the pH is relatively low, the metal oxides A and B both similarly have a large zeta potential (Cl1 ), whereas when the pH is relatively high, the metal oxide A has a relatively small zeta potential, though the sign thereof is the same as that of the zeta potential of the metal oxide B (Cl") Accordingly, in the state where the pH is relatively high (Cl"), that is, at a pH where the zeta potential of the metal oxide A is smaller than 1/2 of the zeta potential of the metal oxide B, when platinum is loaded by using a tetranitroplatinate solution, the tetranitroplatinate complex ion having a negative charge is selectively drawn by the Coulomb force to the oxide B having a larger positive charge. Similarly, the hexanitroplatinate having a negative charge is also drawn to the oxide B having a large positive charge. On the other hand, the hexaamminerhodium having a positive charge is preferentially deposited on the oxide A from which repulsion by the Coulomb force is relatively small. <pH larger than the isoelectric point of the oxide B> This is considered to be the same as the case where the pH is smaller than the isoelectric point of the oxide A. Accordingly, when the pH is relatively low in this range, the noble metal can be selectively loaded by utilizing the difference between the zeta potentials of the metal oxides A and B. The process of the present invention is described in detail below. The first and second metal oxide supports usable in the process of the present invention can be selected as a combination of metal oxides which differ from each other in the mode of change of the zeta potential due to change of the pH value. For example, each metal oxide is a powder or colloid particle of a metal oxide selected from the group consisting of ceria, zirconia, alumina, titania and silica. The zeta potential property of this metal oxide is known to be generally a value inherent to the metal oxide. However, in the case of using a metal oxide support in the present invention, the zeta potential property can be changed by the surface modification of the metal oxide support, particularly surface modification with an organic compound. The solution containing the first and second metal oxide supports may be any liquid suitable for mixing with a noble metal solution to load the noble metal on the first and second metal oxide supports, for example, water. For the purpose of adjusting the pH of the solution, any acid or alkali may be added to the solution. Examples of the acid which can be used include mineral acids such as nitric acid and hydrochloric acid, and examples of the alkali which can be used include aqueous ammonia and sodium hydroxide. The pH of the solution can be adjusted by adding an acid or an alkali to the solution while measuring the pH of the solution by a pH meter. Alternatively, the pH may also be adjusted by a method wherein the amount of an acid or alkali necessary for the adjustment of pH is measured by using a previously sampled solution, the amount of an acid or alkali necessary for the entire solution is determined based on the measured value, and then an acid or alkali is added to the entire solution in the determined amount. The noble metal solution usable for the present invention may be any noble metal solution containing a noble metal ion or complex ion having a positive or negative charge, particularly, a noble metal complex solution containing a noble metal complex ion, or a noble metal nitrate solution. The noble metal may be, for example, platinum, rhodium or palladium. The drying and firing of the metal oxide support having supported thereon the noble metal may be performed by any method at any temperature. For example, the metal oxide support may be dried by placing the metal oxide support in an oven at 12O0C. The thus-dried metal oxide support can be fired to obtain an exhaust gas purifying catalyst. The firing can be performed at a temperature generally employed in the synthesis of metal oxides, for example, at a temperature of 300 to l,100°C. The exhaust gas purifying catalyst of the present invention wherein the amount of the noble metal supported gradually decreases from the outer side surface toward the inner side of the catalyst-support layer or pellet and, at the same time, the amount of the noble metal supported per unit surface area of the first metal oxide support is larger than the amount of the noble metal supported per unit surface area of the second metal oxide support, can be produced by the process of the present invention, that is, by using, in the step (a) of the process of the present invention, a catalyst-support layer or pellet in which first and second metal oxides are stacked or mixed. In the case of a conventional process of previously loading different noble metals on first and second metal oxide support powders and then mixing the first and second support powders each supporting thereon a noble metal, as shown in Fig. 2 (b) , the noble metal may be selectively loaded on the first metal oxide support, but the noble metal is supported at a substantially uniform concentration across the thickness of the catalyst- support layer or pellet. On the other hand, in the case of the process of the present invention wherein a noble metal is selectively loaded on a previously shaped catalyst-support layer or pellet, as shown in Fig. 2 (a) , the noble metal can be selectively loaded on the first metal oxide support and, at the same time, the amount of the noble metal supported can be gradually decreased from the outer side surface toward the inner side of the catalyst-support layer or pellet. In the exhaust gas purifying catalyst of the present invention, as the first and second metal oxide supports and the noble metal, the above described supports and noble metals, with respect to the process of the present invention, can be used. The present invention is described below by referring to Examples, but the present invention is not limited thereto. EXAMPLES <Example 1> Particulate ceria and particulate zirconia (molar ratio = 3:2) were dispersed in water to obtain a liquid dispersion, and the pH of this liquid dispersion was adjusted to 6.5 which was between the isoelectric point of the particulate ceria (pH 7.2) and the isoelectric point of the particulate zirconia (pH 4.7) . While maintaining this pH, a tetranitroplatinate (Pt (NO2) 42~) solution which is a strongly acidic solution was added to the liquid dispersion in an amount of giving a platinum content of 1 wt% based on the total weight of the particulate ceria and particulate zirconia. Thereafter, the obtained liquid dispersion was dried at 120°C for 5 hours and fired at 500°C to obtain a catalyst powder. For the activity evaluation of catalyst, the obtained catalyst powder was shaped into 1 mm-cubic pellets. <Comparative Example 1> A catalyst powder was obtained in the same manner as in Example 1 except that the pH was not adjusted. The pH of the liquid dispersion was about 1.5 when the tetranitroplatinate solution as a strongly acidic solution was added thereto. For the activity evaluation of catalyst, the obtained catalyst powder was shaped into 1 mm-cubic pellets. <Performance Evaluation of Catalysts of Example 1 and Comparative Example 1> The catalyst pellets were made to endure firing at 9000C for 3 hours in air. Thereafter, a rich gas and a lean gas each having the composition shown in Table 1 below were alternately passed to the catalyst pellets at a cycle of 1 Hz and, by elevating the temperature of these rich/lean gases, the temperatures where the purification ratios of HC, CO and NO reached 50% (50% purification temperature) were determined. <Table 1> Table 1: Composition of Evaluation Gas

Fig. 3 shows the obtained 50% purification temperatures. As is apparent from Fig. 3, for all of HC, CO and NO, the 50% purification temperatures in Example 1 were lower than that in Comparative Example 1. This reveals that the catalyst of Example 1 exerts good activity from a relatively low temperature as compared with the catalyst of Comparative Example 1. <Example 2> A ceria-zirconia composite oxide support (ceria: zirconia = 3:2 (molar ratio)) was dispersed in water to obtain a liquid dispersion, and the pH of this liquid dispersion was adjusted to 6.5 which is between the isoelectric point of ceria (pH 7.2) and the isoelectric point of zirconia (pH 4.7) . While maintaining this pH, a hexaamminerhodium (Rh (NH3) 63+) solution was added to the liquid dispersion in an amount of giving a rhodium content of 0.5 wt% based on the total weight of the ceria-zirconia composite oxide support. Thereafter, the obtained liquid dispersion was dried at 1200C for 5 hours and fired at 500°C to obtain a catalyst powder. For the activity evaluation of catalyst, the obtained catalyst powder was shaped into 1 mm-cubic pellets. <Comparative Example 2> A catalyst powder was obtained in the same manner as in Example 2 except that the pH was not adjusted and rhodium nitrate was used in place of the hexaammine¬ rhodium. The pH of the liquid dispersion was about 1 after the rhodium nitrate solution as a strongly acidic solution was added thereto. For the activity evaluation of catalyst, the obtained catalyst powder was shaped into 1 mm-cubic pellets. <Performance Evaluation of Catalysts of Example 2 and Comparative Example 2> The 50% purification temperatures for HC, CO and NO were determined in the same manner as in Example 1 and Comparative Example 1. Fig. 4 shows the obtained 50% purification temperatures. As apparent from Fig. 4, for all of HC, CO and NO, the 50% purification temperatures in Example 2 were lower than that in Comparative Example 2. This reveals that the catalyst of Example 2 exerts good activity from a relatively low temperature as compared with the catalyst of Comparative Example 2. <Example 3> A ceria-zirconia composite oxide support (ceria: zirconia (molar ratio) = 3:2) was dispersed in water to obtain a liquid dispersion, and the pH of this liquid dispersion was adjusted to 4 which is lower than the isoelectric point of zirconia (pH 4.7) and in which the zeta potential of ceria (isoelectric point: pH 7.2) becomes larger than 2 times the zeta potential of zirconia. While maintaining this pH, a tetranitroplatinate (Pt (NO2) 42~) solution was added to the liquid dispersion in an amount of giving a platinum content of 1 wt% based on the total weight of the ceria- zirconia composite oxide support. Thereafter, the obtained liquid dispersion was dried at 1200C for 5 hours and fired at 500°C to obtain a catalyst powder. For the activity evaluation of catalyst, the obtained catalyst powder was shaped into 1 mm-cubic pellets. <Comparative Example 3> A catalyst powder was obtained in the same manner as in Example 3 except that the pH was not adjusted. The pH of the liquid dispersion was about 1.5 after the tetranitroplatinate solution as a strongly acidic solution was added thereto. For the activity evaluation of catalyst, the obtained catalyst powder was shaped into a 1 mm-square pellet. <Performance Evaluation of Catalysts of Example 3 and Comparative Example 3> The 50% purification temperatures for HC, CO and NO were determined in the same manner as in Example 1 and Comparative Example 1. Fig. 5 shows the obtained 50% purification temperatures. As is apparent from Fig. 5, for all of HC, CO and NO, the 50% purification temperatures in Example 3 were lower than that in Comparative Example 3. This reveals that the catalyst of Example 3 exerts good activity from a relatively low temperature as compared with the catalyst of Comparative Example 3. <Example 4> Particulate ceria (isoelectric point: pH 7.2) and particulate zirconia (isoelectric point: pH 4.7) were dispersed in water to obtain a liquid dispersion (ceria: zirconia (molar ratio) = 3:2), and the pH of this liquid dispersion was adjusted to 4. While maintaining this pH, a hexanitroplatinate (Pt(NO2)64~) solution was added to the liquid dispersion in an amount of giving a platinum content of 1 wt% based on the total weight of the particulate ceria and particulate zirconia. Thereafter, the obtained liquid dispersion was dried at 1200C for 5 hours and fired at 500°C to obtain a catalyst powder. For the activity evaluation of catalyst, the obtained catalyst powder was shaped into 1 mm-cubic pellets. <Example 5> A catalyst powder was obtained in the same manner as in Example 4 except for using a tetranitroplatinate (Pt (NO2) 42~) solution in place of the hexanitroplatinate (Pt (NO2) 64~) solution. For the activity evaluation of catalyst, the obtained catalyst powder was shaped into 1 mm-cubic pellets. <Performance Evaluation of Catalysts of Examples 4 and 5> The 50% purification temperatures for HC, CO and NO were determined in the same manner as in Example 1 and Comparative Example 1. However, catalysts were made to endure firing at 9000C for 5 hours in air. Fig. 6 shows the obtained 50% purification temperatures. As apparent from Fig. 6, for all of HC, CO and NO, the 50% purification temperatures in Example 4 were lower than that in Example 5. This reveals that the catalyst of Example 4 exerts good activity from a relatively low temperature as compared with the catalyst of Example 5. After the endurance test, the dispersibility of platinum was measured by the CO pulse adsorption at -2O0C. Fig. 7 shows the results. As seen from Fig. 7, the dispersibility was better in Example 4 than in Example 5. <Example 6> While adjusting the pH of an alkali-stabilized aqueous zirconia sol solution (isoelectric point: pH 3.5) to 5, an acidic-stabilized aqueous ceria sol solution (isoelectric point: pH 8.5) and a tetranitroplatinate (Pt (NO2) 42~) solution were added thereto (CeO2: ZrO2 (molar ratio) = 1:1), platinum content: 1 wt% based on the total weight of ceria and zirconia) . The resulting solution was dried at 12O0C for 24 hours and the dried product was fired at 7000C for 5 hours to obtain a catalyst powder. For the activity evaluation of catalyst, the obtained catalyst powder was shaped into 1 mm-cubic pellets. <Comparative Example 4> A catalyst powder was obtained in the same manner as in Example 6 except that the pH was not adjusted. The pH of the liquid dispersion was about 2 after the tetranitroplatinate solution was added thereto. For the activity evaluation of catalyst, the obtained catalyst powder was shaped into 1 mm-cubic pellets. <Performance Evaluation of Catalysts of Example 6 and Comparative Example 4> The 50% purification temperatures for HC, CO and NO were determined in the same manner as in Example 1 and Comparative Example 1. Fig. 8 shows the obtained 50% purification temperatures. As is apparent from Fig. 8, for all of HC, CO and NO, the 50% purification temperatures were lower in Example 6 than in Comparative Example 4. This reveals that the catalyst of Example 6 exerts good activity from a relatively low temperature as compared with the catalyst of Comparative Example 4. <Example 7> While adjusting the pH of an acid-stabilized aqueous ceria sol solution (isoelectric point: pH 8.5) to 6, an alkali-stabilized aqueous zirconia sol solution (isoelectric point: pH 3.5) and a hexaamminerhodium (Rh(NH3J63+) solution were added thereto (ZrO2:CeO2 (molar ratio = 1:1), rhodium content: 1 wt% based on the total weight of ceria and zirconia) . The resulting solution was dried at 1200C for 24 hours and the dried product was fired at 7000C for 5 hours to obtain a catalyst powder. For the activity evaluation of catalyst, the obtained catalyst powder was shaped into 1 mm-cubic pellets. <Comparative Example 5> A catalyst powder was obtained in the same manner as in Example 7 except that the pH was not adjusted. The pH of the mixed sol was about 9 after the hexaamminerhodium solution was added thereto. For the activity evaluation of catalyst, the obtained catalyst powder was shaped into 1 mm-cubic pellets. Performance Evaluation of Catalysts of Example 7 and Comparative Example 5> The 50% purification temperatures for HC, CO and NO were examined in the same manner as in Example 1 and Comparative Example 1. Fig. 9 shows the obtained 50% purification temperatures. As is apparent from Fig. 9, for all of HC, CO and NO, the 50% purification temperatures in Example 7 were lower than in Comparative Example 5. This reveals that the catalyst of Example 7 exerts good activity, from a relatively low temperature, as compared with the catalyst of Comparative Example 5.