Catalysts for the purification of exhaust gas
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a catalyst for purifying an exhaust gas discharged from an internal engine for automobiles or the like. 
     2. Description of the Related Art 
     There have hitherto been used catalysts for the purification of exhaust gas using zeolite as disclosed in JP-A-1-127044. This catalyst for the purification of exhaust gas has a structure that a first catalyst layer inclusive of a noble metal ingredient generating oxidation reaction is coated onto a monolith carrier and then a second catalyst layer obtained by ion-exchanging zeolite with Cu is formed on the first catalyst layer. When such a catalyst is used in the purification of the exhaust gas, hydrocarbon (HC), carbon monoxide (CO) and nitrogen oxides (NOx) are efficiently purified even in an exhaust gas atmosphere having an oxygen concentration higher than theoretical value (lean-burn atmosphere) to improve the catalytic purification performance. 
     In the catalyst having the above structure, however, the performances after an engine aging treatment are considerably degraded as compared with those of a catalyst having no oxidation catalytic layer as an inner layer and can not provide satisfactory purification activity. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the invention to provide a novel catalyst for the purification of exhaust gas developing a satisfactory purification performance in lean-burn atmosphere after an engine aging treatment. 
     According to the invention, there is the provision of a catalyst for the purification of exhaust gas comprising a honeycomb monolith carrier, a first coating layer formed on the carrier and composed mainly of activated alumina containing at least one noble metal selected from platinum, palladium and rhodium, a second coating layer formed on the first coating layer and composed mainly of activated alumina containing no noble metal, and a third coating layer formed on the second coating layer and composed mainly of zeolite powder ion-exchanged with a metal such as Cu or Co. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the exhaust gas purification catalyst according to the invention, the first coating layer is composed mainly of activated alumina containing at least one noble metal selected from platinum, palladium and rhodium and induces an oxidation catalytic reaction in the lean-burn atmosphere to purify HC and CO. The second coating layer is composed mainly of activated alumina containing no noble metal ,and has not only a function of insulating heat of oxidation reaction induced in the first coating layer to prevent the temperature rising of the third coating layer during the engine aging treatment and hence control the deterioration of durability but also a function of controlling the deterioration of the purification performance due to the movement of Cu or Co as an active ingredient in the third ingredient from active site of zeolite to the first coating layer through the engine aging treatment. The third coating layer is composed mainly of zeolite powder ion-exchanged with Cu or Co and has a conversion performance of NOx in the lean-burn atmosphere of gas discharged from the engine containing a great amount of oxygen. 
     As the zeolite powder in the third coating layer, use may be made of ZSM-5 zeolite, mordenite, ferrierite and the like. In the second coating layer, the activated alumina can be carried with a metal other than the above noble metal, i.e. rare earth metals such as cerium, lanthanum and the like and zirconium, barium, calcium and potassium. In the latter case, the amount of the metal carried is within a range of 1-15% by weight. 
     The following examples are given in illustration of the invention and are not intended as limitations thereof. 
     EXAMPLE 1 
     A solution of dinitrodiammineplatinum is added to activated alumina powder composed mainly of γ-alumina so as to have a platinum content of 2.0% by weight per 1000 g of the powder with stirring, dried in an oven at 150° C. for 3 hours and fired in air at 400° C. for 2 hours. In a ball mill ]pot are charged 1500 g of the platinum-carried activated alumina, 800 g of activated alumina composed mainly of γ-alumina, 460 g of 10% by weight HNO 3  (nitric acid) and 1840 g of water, which are pulverized for 8 hours to obtain a slurry. The thus obtained slurry is applied onto a honeycomb monolith carrier (1.7 L, 400 cells), dried, and fired in air at 400° C. for 2 hours. In this case, the amount of the slurry applied is set to be 50 g/L after the firing. 
     Then, 2000 g of activated alumina composed mainly of γ-alumina, 400 g of 10% by weight of nitric acid and 1600 g of water are charged in a ball mill pot and pulverized for 8 hours to obtain a slurry. The thus obtained slurry is applied in an amount of 25 g/L after the firing, dried and fired in air at 400° C. for 2 hours. 
     Furthermore, 5.2 kg of 0.2 mol/1 of copper nitrate or copper acetate is mixed with 2 kg of zeolite powder with stirring and filtered. After the mixing and filtering are repeated 3 times, the mixture is dried and fired to prepare zeolite powder ion-exchanged with Cu. In a porcelain ball mill are charged 1890 g of the zeolite powder ion-exchanged with Cu, 11.50 g of silica sol (solid content: 20%) and 1100 g of water, which are pulverized to obtain a slurry. The thus obtained slurry is applied in an amount of 200 g/L after the firing, dried and fired in air at 400° C. for 2 hours to obtain a catalyst No. 1. 
     EXAMPLE 2 
     A catalyst No. 2 is obtained by the same method as in Example 1 except that Co is used instead of Cu as an ion-exchanging metal. 
     EXAMPLE 3 
     A catalyst No. 3 is obtained by the same method as in Example 1 except that palladium is used instead of platinum. 
     EXAMPLE 4 
     A catalyst No. 4 is obtained by the same method as in Example 1 except that platinum and rhodium are used instead of platinum as follows. 
     A solution of dinitrodiammineplatinum is added to activated alumina powder composed mainly of γ-alumina so as to have a platinum content of 1.63% by weight per 1000 g of the powder with stirring, dried and fired in same manner as in Example 1. Furthermore, a solution of rhodium nitrate is added to the same activated alumina powder as mentioned above so as to have a rhodium content of 1% by weight per 1000 g of the powder with stirring, dried and fired in same manner as in Example 1. In a ball mill pot are charged 1400 g of the platinum-carried activated alumina, 434 g of the rhodium-carried activated alumina, 936 g of cerium oxide and 2221 g of acidic boehmite sol (sol obtained by adding 10% by weight of nitric acid to a suspension of 10% by weight of boehmite alumina), which are pulverized for 8 hours to obtain a slurry. The thus obtained slurry is applied onto a honeycomb monolith carrier (1.7 L, 400 cells), dried, and fired in air at 400° C. for 2 hours. In this case, the amount of the slurry applied is set to be 50 g/L after the firing. 
     EXAMPLE 5 
     The same platinum-carried activated alumina as in Example 1 is coated onto the same honeycomb monolith carrier as in Example 1. 
     Then, 2000 g of cerium-carried activated alumina powder containing 3% by weight of Ce, 400 g of 10% by weight of nitric acid and 1600 g of water are charged in a ball mill pot and pulverized for 8 hours to obtain a slurry. The thus obtained slurry is applied in an amount of 25 g/L after the firing, dried and fired in air at 400° C. for 2 hours. 
     Furthermore, the same zeolite ion-exchanged with Cu as in example 1 is applied to obtain a catalyst No. 5. 
     EXAMPLE 6 
     The same platinum-carried activated alumina as in Example 1 is coated onto the same honeycomb monolith carrier as in Example 1. 
     Then, 2000 g of cerium and lanthanum-carried activated alumina powder containing 3% by weight of Ce and 3% by weight of La, 400 g of 10% by weight of nitric acid and 1600 g of water are charged in a ball mill pot and pulverized for 8 hours to obtain a slurry. The thus obtained slurry is applied in an amount of 25 g/L after the firing, dried and fired in air at 400° C. for 2 hours. 
     Furthermore, the same zeolite ion-exchanged with Cu as in example 1 is applied to obtain a catalyst No. 6. 
     EXAMPLE 7 
     A catalyst No. 7 is obtained in the same manner as in Example 3 except that palladium and rhodium are used instead of platinum in Example 1 as follows. 
     A solution of palladium nitrate is added to activated alumina powder composed mainly of γ-alumina so as to have a palladium content of 2.00% by weight per 1000 g of the powder with stirring, dried in an oven at 150° C. for 3 hours and fired in air at 400° C. for 2 hours to prepare a palladium-carried activated alumina. Then, a solution of rhodium nitrate is added to activated alumina powder composed mainly of γ-alumina so as to have a rhodium content of 1.0% by weight per 1000 g of the powder with stirring, dried and fired in the same manner as described above to prepare a rhodium-carried activated alumina. Thereafter, 1400 g of the palladium-carried activated alumina, 434 g of the rhodium-carried activated alumina, 936 g of cerium oxide and 2221 g of acidic boehmite sol (sol obtained by adding 10% by weight of nitric acid to a suspension of 10% by weight of boehmite alumina) are charged in a ball mill pot and pulverized for 8 hours to obtain a slurry. The thus obtained slurry is applied onto a honeycomb monolith carrier (1.7 L, 400 cells), dried, and fired in air at 400° C. for 2 hours. In this case, the amount of the slurry applied is set to be 50 g/L after the firing. 
     COMPARATIVE EXAMPLE 1 
     A catalyst No. 101 is prepared in the same manner as in Example 1 except that zeolite is used instead of the zeolite ion-exchanged with Cu in Example 1. 
     COMPARATIVE EXAMPLE 2 
     A catalyst No. 102 is prepared in the same manner as in Comparative Example 1 except that the second coating layer of activated alumina is omitted. 
     COMPARATIVE EXAMPLE 3 
     A catalyst No. 103 is prepared in the same manner as in Example 4 except that the second coating layer of activated alumina is omitted. 
     Test Example 
     Each of the catalysts in Examples 1-7 and Comparative Examples 1-3 is cut into a specimen having a size of 120 cc, which is placed in a convertor for test and aged by a gas discharged from an engine under the following conditions. Then, the catalytic activity is evaluated with respect to the above aged specimen under the following evaluation conditions to obtain results as shown in Table 1. 
     
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Aging conditions                                                          
Engine displacement                                                       
                   2000 cc                                                
Aging temperature  550° C.                                         
Aging time         50 hours                                               
Composition of emission gas at inlet                                      
                   CO: 0.4-0.6%                                           
                   O.sub.2 : 0.5 ± 0.1%                                
                   NOx: 1500 ppm                                          
                   HC: 1000 ppm                                           
                   CO.sub.2 : 14.9% ± 0.1%                             
Evaluation conditions                                                     
Catalyst volume    0.12 L                                                 
Apparatus          apparatus for evaluation                               
                   of exhaust model gas                                   
                   (use of a bomb as gas)                                 
Inlet temperature of catalyst                                             
                   300° C.                                         
Space velocity     about 20000 h.sup.-1                                   
A/F = model gas corresponding to 18.0                                     
HC = 2500 ppm (converted to Cl)                                           
NOx = 500 PPM                                                             
CO = 2000 ppm                                                             
CO.sub.2 = 14.0%                                                          
O.sub.2 = 4.5%                                                            
H.sub.2 O = 10%                                                           
N.sub.2 = balance                                                         
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                       TABLE 1                                                     
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         Purification ratio at 400° C.                             
         (%)                                                              
Catalyst No.                                                              
           HC           CO      NO.sub.x                                  
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No. 1      81           72      35                                        
No. 2      83           75      33                                        
No. 3      79           71      37                                        
No. 4      81           75      36                                        
No. 5      84           74      37                                        
No. 6      80           73      37                                        
No. 7      79           74      36                                        
No. 101    35           21      23                                        
No. 102    64           78      18                                        
No. 103    61           75      17                                        
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     As mentioned above, the exhaust gas purification catalyst according to the invention comprises a first coating layer of activated alumina containing at least one noble metal of Pt, Pd and Rh, a second coating layer of activated alumina containing no noble metal and a third coating layer of zeolite powder ion-exchanged with Cu or Co and has an effect of developing a satisfactory purification performance in the lean-burn atmosphere after the engine aging treatment.