Abstract:
A process includes the step of bringing exhaust gases into contact with a catalyst, wherein the catalyst includes a support including a porous substance, at least one NO x  adsorbent loaded on the support, and selected from the group consisting of alkaline-earth metals, rare-earth elements and alkali metals, and at least one element loaded on the support, and selected from the group consisting of copper and cobalt. The process enables the catalyst to exhibit superb NO x  purifying performance even after a durability test and to stably and efficiently purify NO x  during the fuel-lean side (i.e., the oxygen-rich atmospheres) driving.

Description:
This is a continuation in part of application Ser. No. 08/379,215, filed Jan. 27, 1995, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to a process for purifying exhaust gases. More particularly, it relates to the process which can efficiently purify nitrogen oxides (NO x ) in the exhaust gases whose oxygen concentrations are at the stoichiometric point or more than required for oxidizing carbon monoxide (CO) and hydrocarbons (HC) therein. 
     2. Description of Related Art 
     As catalysts for purifying automotive exhaust gases, there have been employed 3-way catalysts so far which oxidize CO and HC and reduce NO x  to purify the exhaust gases. For example, the 3-way catalysts have been known widely which comprise a heat resistant support formed of cordierite, a catalyst carrier layer formed of gamma-alumina and disposed on the support, and a noble metal catalyst ingredient selected from the group consisting of Pt, Pd and Rh and loaded on the catalyst carrier layer. 
     The purifying performance of the 3-way catalysts for purifying exhaust gases depends greatly on the air-fuel ratio A/F of automotive engine. For instance, when the air-fuel weight ratio is larger than 14.6, i.e., when the fuel concentration is low (or on the fuel-lean side), the oxygen concentration is high in exhaust gases. Accordingly, the oxidation reactions purifying CO and HC are active, but the reduction reactions purifying NO x  are inactive. On the other hand, when the air-fuel ratio is smaller than 14.6, i.e., when the fuel concentration is higher (or on the fuel-rich side), the oxygen concentration is low in exhaust gases. Accordingly, the oxidation reactions are inactive, but the reduction reactions are active. 
     Moreover, when driving automobiles, especially when driving automobiles in urban areas, the automobiles are accelerated and decelerated frequently. Consequently, the air-fuel ratio varies frequently in the range of from the values adjacent to the stoichiometric point (or the theoretical air-fuel ratio: 14.6) to the fuel-rich side. In order to satisfy the low fuel consumption requirement during the driving conditions such as in the above-described urban areas, it is necessary to operate the automobiles on the fuel-lean side where the air-fuel mixture containing oxygen as excessive as possible is supplied to the engines. Hence, it has been desired to develop a catalyst which is capable of adequately purifying NO x  even on the fuel-lean side (i.e., in the oxygen-rich atmospheres). 
     In view of the aforementioned circumstances, the applicant et al. of the present invention applied for a Japanese Patent for a novel catalyst under Japanese Patent Applicaton No. 4-130,904 (Japanese Unexamined Patent Publication (KOKAI) No. 5-317,652). On this catalyst, there are loaded an alkaline-earth metal oxide and Pt. In the catalyst, during the fuel-lean side (i.e., in the oxygen-rich atmospheres) driving, NO x , which includes NO in an amount of about 90% by volume and the balance of NO 2  etc., is adsorbed on the alkaline-earth metal elements. For instance, the NO is oxidized to NO 2  by the Pt, and the resulting NO 2  is adsorbed on the alkaline-earth metal elements. When the air-fuel mixture varies from the stoichiometric point to the fuel-rich states, the adsorbed NO x  is released from the alkaline-earth metal elements, and it is reacted with the reducing gas like HC by the action of the Pt. Thus, NO x  is reduced and purified to N 2 . As a result, the catalyst exhibits superb NO x  purifying performance during the fuel-lean side (i.e., in the oxygen-rich atmospheres) driving. 
     The catalyst proposed in Japanese Unexamined Patent Publication (KOKAI) No. 5-317,652 is believed to provide the advantageous effect as follows: the alkaline-earth metal oxides, for example, barium oxide loaded on the support, react with NO x  to produce nitrates, e.g., Ba(NO 3 ) 2 . Thus, NO x  is adsorbed on the support of the catalyst in the form of the alkaline-earth metal nitrates. 
     However, the exhaust gases usually contain SO 2  which is produced by burning sulfur (S) contained in the fuel. Further, the catalyst ingredient (e.g., Pt or the like) oxidizes SO 2  to SO 3  in the oxygen-rich atmospheres (i.e., on the fuel-lean side). Still further, SO 3  reacts readily with water vapor also contained in the exhaust gases to produce sulfuric acid. It has been revealed that the resulting sulfuric acid reacts with the alkaline-earth metal elements to produce alkaline-earth metal sulfites and alkaline-earth metal sulfates, thereby poisoning and degrading the alkaline-earth metal elements. Specifically, when the alkaline-earth metal elements are turned into the sulfites and sulfates, they hardly adsorb No x  thereon. As a result, the catalyst proposed in Japanese Unexamined Patent Publication (KOKAI) No. 5-317,652 might suffer from a drawback in that it is deteriorated in terms of the NO x  purifying performance after it is subjected to a durability test. 
     SUMMARY OF THE INVENTION 
     The present invention has been developed in view of the aforementioned circumstances. It is therefore an object of the present invention to inhibit NO x  adsorbents (e.g., alkaline-earth metals or the like) from being poisoned and degraded by sulfur, thereby purifying NO x  efficiently in exhaust gases of oxygen-rich atmospheres even after a durability test. 
     In accordance with a first aspect of the present invention, there is proposed a process, which can carry out the aforementioned object, for purifying an exhaust gas by simultaneously removing carbon monoxide, hydrocarbons and nitrogen oxides contained in the exhaust gas, bringing the exhaust gas into contact with an exhaust gas purifying catalyst comprising: 
     a support including a porous substance; 
     at least one NO x  adsorbent loaded on the support, and selected from the group consisting of alkaline-earth metals, rare-earth elements an alkali metals; and 
     at least one element loaded on the support, and selected from the group consisting of copper and cobalt; 
     whereby the majority of the nitrogen oxides (NO x ) in the exhaust gas are adsorbed into the NO x  adsorbent on the support under an oxygen-rich atmosphere in which oxidizing elements are excess of stoichiometric point which is required for oxidizing non-oxidizing elements in the exhaust gas, and the nitrogen oxides (NO x ) adsorbed into the NO x  adsorbent on the support are released and purified by the reaction with the carbon monoxide and hydrocarbons in the exhaust gas under stoichiometric point air-fuel ratio or a fuel-rich atmosphere in which oxidizing elements are below stoichiometric point which is required for oxidizing non-oxidizing elements in the exhaust gas. 
     The exhaust gas purifying catalyst is preferably free of all platinum group components. In particular, the exhaust gas purifying catalyst is preferably free of platinum, palladium and rhodium. 
     Although the process according to the first aspect of the present invention may exhibit, compared to the process employing the catalyst disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 5-317,652 and including Pt and alkaline-earth metal loaded thereon, low maximum NO x  conversion in the fuel-lean side (i.e., in the oxygen-rich atmospheres) driving, it can exhibit NO x  conversion less likely to deteriorate in the fuel-lean side (i.e., in the oxygen-rich atmospheres) driving even after the employed catalyst is subjected to a durability test. Thus, in accordance with the first aspect of the present invention, the present process can purify NO x  for a long period of time. The advantageous effect is believed to be effected by the following mechanism. 
     Cu and Co have lower oxidation activity than Pt does. Accordingly, SO 2  is flowed to the downstream side without being oxidized. As a result, the NO x  adsorbent little reacts with SO 3  to produce its sulfate, and can stably adsorb NO x  thereon. Although the Cu and Co do not have oxidation activity as high as that of the Pt, they have oxidation activity which can oxidize NO, one of the major components of NO x , to produce NO 2  which is likely to be adsorbed on the NO x  adsorbent. In other words, NO is hardly adsorbed on the NO x  adsorbent as it is, but, after it is oxidized to NO 2  by the Pt or the like, it can be adsorbed on the NO x  at last. The Cu and Co have the oxidizing catalyst activity so that they carry out the reaction: NO+O=NO 2 . Thus, NO x  adsorbent can securely adsorb NO x  thereon on the fuel-lean side. Then, in the running conditions of vehicle engine ranging from the driving at the stoichiometric point to the fuel-rich side (i.e., in the oxygen-lean atmospheres) driving, the NO x  adsorbent releases NO x  which has been adsorbed thereon in the fuel-lean side (i.e., in the oxygen-rich atmospheres) driving. The catalytic action of the Cu and Co reduces and purifies the thus released NO x  to N 2 , thereby reacting NO x  with CO and HC in the exhaust gases and simultaneously oxidizing and purifying the CO and HC. By this type of mechanism, the NO x  adsorbing ability of the employed catalyst can be kept for a prolonged period of time, and the high activity thereof can be maintained as well. 
     As for the porous substance, the following can be listed as examples: alumina, zeolite, zirconia, silica-alumina, silica and the combinations of them. These porous substances themselves can constitute a support, or they can be coated on a surface of a honeycomb body made from cordierite, heat resistant metal, etc. 
     As for a loading amount of the copper, it is preferable to load the copper in an amount of from 0.1 to 3.0 moles with respect to 1 liter of the support. As for a loading amount of the cobalt, it is preferable to load the cobalt in an amount of from 0.1 to 3.0 moles with respect to 1 liter of the support. 
     As for the NO x  adsorbent, it is possible to employ at least one element selected from the group consisting of alkaline-earth metals, rare-earth elements and alkali metals. The alkaline-earth metal can be Ba, Sr, Ca and the like. The rare-earth element can be La, Y, Ce and the like. The alkali metal can be Li, K, Na and the like. Unless otherwise specified, the term, &#34;rare-earth elements,&#34; herein includes not only the chemical elements with atomic numbers 58 through 71, but also  39  Y and  57  La. 
     As for a loading amount of the NO x  adsorbent, it is preferable to load the NO x  adsorbent in an amount of from 0.1 to 1.0 mole, further preferably from 0.1 to 0.3 moles, with respect to 1 liter of the support. 
     In the present process according to the first aspect of the present invention, the Co may possibly react with the alumina or the NO x  adsorbent at about 800° C. to produce certain compounds. When such compounds are produced, the Co as well as the NO x  adsorbent may lose their inherent functions. Consequently, no improvement may possibly be expected in terms of the NO x  conversion after a durability test. 
     In order to overcome this possible drawback, there is further proposed, in accordance with a second aspect of the present invention, a process, which can also carry out the aforementioned object, for purifying an exhaust gas by simultaneously removing carbon monoxide, hydrocarbons and nitrogen oxides contained in the exhaust gas, 
     bringing the exhaust gas into contact with an exhaust gas purifying catalyst comprising: 
     a cobalt-aluminate support wherein cobalt is dissolved in alumina; and 
     at least one NO x  adsorbent loaded on the cobalt-aluminate support, and selected from the group consisting of alkaline-earth metals, rare-earth elements and alkali metals; 
     whereby the majority of the nitrogen oxides (NO x ) in the exhaust gas are adsorbed into the NO x  adsorbent on the cobalt-aluminate support under an oxygen-rich atmosphere in which oxidizing elements are excess of stoichiometric point which is required for oxidizing non-oxidizing elements in the exhaust gas, and the nitrogen oxides (NO x ) adsorbed into the NO x  adsorbent on the cobalt-aluminate support are released and purified by the reaction with the carbon monoxide and hydrocarbons in the exhaust gas under stoichiometric point air-fuel ratio or a fuel-rich atmosphere in which oxidizing elements are below stoichiometric point which is required for oxidizing non-oxidizing elements in the exhaust gas. 
     The exhaust gas purifying catalyst is preferably free of all platinum group components. In particular, the exhaust gas purifying catalyst is preferably free of platinum, palladium and rhodium. 
     The present process according to the second aspect of the present invention employs the catalyst which comprises the support including cobalt-aluminate with the NO x  adsorbent (e.g., Ba etc.) loaded thereon. With this arrangement, when carrying out the present process, it is possible to inhibit the Co from reacting with the alumina in the employed catalyst and accordingly to inhibit the NO x  conversion from deteriorating after a durability test. 
     Although one may foresee a possibility that the inherent catalytic action of the Co, for instance, its action of oxidizing NO in the exhaust gases and reducing the released NO 2  degrades on certain occasions, it is possible to avoid the possibility by controlling the dissolving amount of the Co. When the Co dissolving amount is thus controlled, it is possible to produce an advantageous effect which is equivalent to that produced by loading Co in the form of the simple substance. For example, it is preferable to dissolve the Co in the alumina in an amount of from 0.3 to 0.7 moles, further preferable in an amount of from 0.3 to 0.5 moles, with respect to 1 liter of the cobalt-aluminate support. 
     In addition, in accordance with the second aspect of the present invention, though reasons are still under investigation, the following advantageous effects are provided. That is, since the catalyst comprises the cobalt-aluminate support, the NO x  adsorbent, such as Ba and the like, can be inhibited from reacting with the alumina, and accordingly its NO x  adsorbing capability can be inhibited from degrading. 
     Likewise, in the second aspect of the present invention, the cobalt-aluminate itself can constitute a support, or it can be coated on a surface of a honeycomb body made from cordierite, heat resistant metal, etc. 
     Similarly, in the second aspect of the present invention, aforementioned specific alkaline-earth metals, rare-earth elements, alkali metals and the combinations thereof can constitute the NO x  adsorbent. The NO x  adsorbent is also preferably loaded in the amount of from 0.1 to 1.0 mole, further preferably from 0.1 to 0.3 moles, with respect to 1 liter of the cobalt-aluminate support. 
     Thus, in accordance with the first and second aspects of the present invention, the employed catalyst can be inhibited from suffering from the deterioration resulting from the NO x  adsorbent which is poisoned and degraded by the sulfur contained in the exhaust gases. That is, the first and second aspects of the present invention enable the employed catalyst to exhibit superb NO x  purifying performance even after a durability test and to stably and efficiently purify NO x  during the fuel-lean side (i.e., oxygen-rich atmospheres) driving. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Having generally described the present invention, a further understanding can be obtained by reference to the specific preferred embodiments which are provided herein for purposes of illustration only and are not intended to limit the scope of the appended claims. 
     The present invention will be hereinafter described in detail with reference to the preferred embodiments along with comparative examples. Unless otherwise specified, the term, &#34;parts,&#34; hereinafter means &#34;parts by weight.&#34; 
     First Preferred Embodiment 
     (Preparation of Catalyst) 
     100 parts of an alumina powder, 70 parts of alumina sol containing alumina in an amount of 10% by weight, 15 parts of an alumina nitrate aqueous solution containing aluminum nitrate in an amount of 40% by weight, and 30 parts of water were mixed, thereby preparing a slurry for coating. 
     Then, a plurality of honeycomb supports formed of cordierite were immersed into the slurry, and thereafter each of them was blown to blow away the slurry in excess. After the blowing, each of the supports was dried at 80° C. for 20 minutes, and thereafter each of them was calcinated at 600° C. for 1 hour, thereby forming an alumina coating layer. The coating layer was thus coated on the honeycomb support in an amount of 120 grams with respect to 1 liter of the honeycomb support. 
     Each of the honeycomb supports having the coating layer was immersed into a copper nitrate aqueous solution, and each of them was blown to blow away the water droplets in excess. After the blowing, each of the supports was dried at 110° C., and thereafter each of them was calcinated at 600° C. for 1 hour, thereby loading Cu thereon. The loading amount of Cu was 0.1 mole, defined by moles of the metallic component, with respect to 120 grams of the alumina coating layer. 
     Then, each of the honeycomb supports with Cu loaded was immersed into a barium acetate aqueous solution having a predetermined concentration, and thereafter each of them was blown to blow away the water droplets in excess. After the blowing, each of them was dried at 110° C., and thereafter each of them was calcinated at 600° C. for 1 hour, thereby preparing catalysts identified with No. 1 in Table 1 below. The loading amount of Ba was 0.3 moles, defined by moles of the metallic component, with respect to 120 grams of the alumina coating layer. 
     Further, by following the same procedure as described above, two catalysts identified with Nos. 2 and 3 in Table 1 were prepared, on catalysts which Cu was loaded in an amount of 0.3 moles and 0.5 moles, defined by its metallic component, with respect to 120 grams of the alumina coating layer, respectively. 
     Furthermore, except that a cobalt acetate aqueous solution was used instead of the copper nitrate aqueous solution, another two catalysts identified with Nos. 4 and 5 in Table 1 were prepared in the aforementioned manner, on catalysts which Co was loaded in an amount of 0.1 mole an 0.3 moles, defined by its metallic component, with respect to 120 grams of the alumina coating layer, respectively. Ba was loaded on each of the catalysts in an identical amount. 
     Moreover, except that Sr, K and La substituted for the Ba, still another three catalysts identified with Nos. 6, 7 and 8 in Table 1 were prepared in the aforementioned manner, on catalysts which Co was loaded in the same amount as that of catalyst No. 5. 
     In addition, the following comparative examples were prepared: 
     except that a platinum dinitrodiammine aqueous solution was used instead of the copper nitrate aqueous solution, comparative catalysts identified with No. 9 in Table 1 were prepared in the same manner as catalyst No. 1, on comparative catalysts which Pt was loaded in an amount of 2.0 grams, defined by its metallic component, with respect to 120 grams of the alumina coating layer; and 
     
                                           TABLE 1__________________________________________________________________________      Cu Loading            Co Loading                  NO.sub.x Adsorbent Loading                              Pt Loading  Amount Amount Amount (mole/liter) Amount      (mole/liter)            (mole/liter)                  Ba Sr K  La (gram/liter)__________________________________________________________________________Catalysts No. 1      0.1   --    0.3                     -- -- -- --  Catalysts No. 2 0.3 -- 0.3 -- -- -- --  Catalysts No. 3 0.5 -- 0.3 -- -- -- --  Catalysts No. 4 -- 0.1 0.3 -- -- -- --  Catalysts No. 5 -- 0.3 0.3 -- -- -- --  Catalysts No. 6 0.5 0.3 -- 0.3 -- -- --  Catalysts No. 7 -- 0.3 -- -- 0.3 -- --  Catalysts No. 8 -- 0.3 -- -- -- 0.3 --  Comp. Catalysts No. 9 -- -- 0.3 -- -- -- 2.0  Comp. Catalysts No. 10 0.1 -- 0.3 -- -- -- 2.0__________________________________________________________________________ 
    
     except that, after loading the Cu in the same manner as catalysts No. 1, a platinum dinitrodiammine aqueous solution was further used in the same manner as comparative catalysts No. 9, comparative catalysts identified with No. 10 in Table 1 were prepared. 
     Note that, in comparative catalysts Nos. 9 and 10, the Ba was loaded in the same amount as those of catalysts Nos. 1 through 5. 
     Table 1 sets forth loaded metals and their loading amounts in catalysts Nos. 1 through 8 as well as in comparative catalysts Nos. 9 and 10. 
     Examination for Purifying Performance 
     Each of catalysts Nos. 1 through 8 as well as comparative catalysts Nos. 9 and 10, and a heat exchanger were disposed in an exhaust line of an actual vehicle equipped with a lean burn engine. The lean burn engine had a displacement of 1.6 liters. The vehicle was driven so as to vary the air-fuel mixture in a pulsating manner. For instance, the air-fuel mixture was varied alternatively from a fuel-lean state whose air-fuel ratio A/F was held at 20 for 2 minutes to a stoichiometric state whose air-fuel ratio A/F was held at 14.6 for 2 minutes. The exhaust gases were introduced into each of the catalysts at a temperature of from 200 to 500° C., and the NO x  conversion was examined at intervals of 50° C. 
     After the aforementioned examination, each of catalysts Nos. 1 through 8 as well as comparative catalysts Nos. 9 and 10 was subjected to a bench test on durability which utilized the same type of engine as above. Namely, each of the catalysts was disposed in an exhaust line of the engine, and then the engine was run at an air-fuel ratio A/F of 18 for 50 hours while adjusting the temperature of the exhaust gas introduced into each of them at 650° C. After this bench test was over, each of the catalysts was again disposed in the exhaust line of the actual vehicle. The vehicle was driven in the same manner as described above, thereby examining the catalysts for the conversion of NO x  after a durability test. In the bench test on durability, a fuel was used which contained sulfur in an amount of 70 ppm. The results of these examinations for the NO x  conversion are summarized in Table 2 below. In Table 2, the initial NO x  conversions are listed in the columns designated at &#34;INC&#34; (i.e., Initial NO x  Conversion), the NO x  conversions after the durability test are listed in the columns designated at &#34;NCDT&#34; (i.e., NO x  Conversion after a Durability Test), and the values are expressed in %. 
     It is appreciated from Table 2 that, concerning the initial NO x  conversion, the exhaust-gases-purifying process of the First Preferred Embodiment employing catalysts Nos. 1 through 8 was inferior to the comparative exhaust-gases-purifying process employing comparative catalysts Nos. 9 and 10. However, it is also understood from Table 2 that, concerning the deterioration of the NO x  conversion after the durability test, the exhaust-gases-purifying process of the First Preferred Embodiment exhibited the NO x  conversion, which was far less likely to deteriorate than the NOX conversion exhibited by the comparative exhaust-gases-purifying process, after the durability test. Thus, concerning the NO x  conversion after the durability test, the exhaust-gases-purifying process of the First Preferred Embodiment was superior to the comparative exhaust-gases-purifying process. 
     Moreover, the temperature window was placed at higher-temperature sides by the exhaust-gases-purifying process of the First Preferred Embodiment than by the comparative exhaust-gases-purifying process. Accordingly, the exhaust-gases-purifying process of the First Preferred Embodiment enables to dispose an exhaust-gases-purifying catalyst directly below a vehicle engine. It has been difficult so far to dispose and use an exhaust-gases-purifying catalyst in such a manner. 
     
                                           TABLE 2__________________________________________________________________________      200° C.             250° C.                     300° C.                            350° C.                                    400° C.                                           450° C.                                                   500° C.      INC         NCDT             INC NCDT                     INC                        NCDT                            INC NCDT                                    INC                                       NCDT                                           INC NCDT                                                   INC                                                      NCDT__________________________________________________________________________Catalysts No. 1      -- --  2   2    8  7  32  26  50 40  38  31  30 25  Catalysts No. 2 -- -- 5 5 16 14 40 31 60 51 48 42 33 28  Catalysts No. 3 -- -- 8 8 18 16 42 33 56 45 42 34 31 27  Catalysts No. 4 -- -- -- --  2  0 20 12 40 30 51 46 46 39  Catalysts No. 5 -- -- -- --  4  2 25 14 43 34 61 50 48 36  Catalysts No. 6 -- -- 3 1 -- -- 22 11 40 30 58 49 46 34  Catalysts No. 7 -- -- 2 -- -- -- 24 12 44 36 64 53 47 36  Catalysts No. 8 -- -- -- -- -- -- 18 9 36 24 50 41 40 30  Comp. Catalysts No. 9 14 10 65      34  82 42 58 31 12  4 -- -- -- --                                                       Comp.                                                      Catalysts No.                                                      10 12  8 66                                                      32  86 43 60                                                      32 18  8 -- --                                                      -- --__________________________________________________________________________ (Note) &#34;INC&#34; stands for Initial NO.sub.x Conversion. &#34;NCDT&#34; stands for NO.sub.x Conversion after a Durability Test. 
    
     Second Preferred Embodiment 
     The honeycomb support having the alumina coating layer and prepared in the same manner as in the First Preferred Embodiment was also employed to produce catalysts to be employed by the exhaust-gases-purifying process of the Second Preferred Embodiment. 
     Each of the honeycomb supports having the alumina coating layer was immersed into a cobalt nitrate aqueous solutions having a predetermined concentration. After taking each of the supports out of the aqueous solution, each of them was blown to blow away the water droplets in excess. After the blowing, each of the supports was dried, and thereafter each of them was calcinated at 800° C. for 3 hours, thereby forming Co-aluminate in which Co was dissolved in the alumina-coating layer. Thus, a plurality of honeycomb supports were prepared which had the Co-aluminate thereon. 
     Then, each of the honeycomb supports having the Co-aluminate thereon was immersed into a barium acetate aqueous solution. After taking each of the supports out of the aqueous solution, each of them was blown to blow away the water droplets in excess. After the blowing, each of the supports was dried, and thereafter each of them was calcinated at 500° C. for 3 hours, thereby loading Ba thereon. Catalysts identified with Nos. 11 through 15 in Table 3 below were thus prepared. Table 3 also sets forth the loading amounts of Co and Ba on each of the catalysts. 
     Except that a potassium acetate aqueous solution was used instead of the barium acetate aqueous solution, catalysts identified with No. 16 in Table 3 were prepared in the same manner as the catalysts identified with Nos. 11 through 15. The loading amounts of Co and K are also set forth in Table 3. 
     Except that either Sr or La substituted for Ba, two kinds of catalysts identified with Nos. 17 and 18 in 
     
                                           TABLE 3__________________________________________________________________________                                  Conversionafter  Co Loading NO.sub.x Adsorbrent Loading Initial Conversion Durability                                 Test  Amount Amount (mole/liter) (%) (%)      (mole/liter)            Ba K  Sr La NO.sub.x                           HC CO NO.sub.x                                    HC CO__________________________________________________________________________Catalysts No. 11      0.1   0.3               -- -- -- 41 94 99 34 82 86  Catalysts No.  12 0.3 0.3 -- -- -- 65 96 100 57 88 90  Catalysts No.  13 0.5 0.3 -- -- -- 89 97 100 68 91 92  Catalysts No.  14 0.7 0.3 -- -- -- 78 98 100 62 94 93  Catalysts No.  15 0.9 0.3 -- -- -- 75 98 100 48 86 88  Catalysts No.  16 0.5 -- 0.3 -- -- 67 94 100 60 84 90  Catalysts No.  17 0.5 -- -- 0.3 -- 71 94 100 58 88 90  Catalysts No.  18 0.5 -- -- -- 0.3 65 99 100 52 94 96  Comp. Catalysts No. 19 0.1 0.3 -- -- -- 50 94 100 30 80 82  Comp. Catalysts No. 20 0.3 0.3 -- -- -- 72 96 100 42 81 88  Comp. Catalysts No. 21 0.5 0.3 -- -- -- 92 99 100 48 83 90__________________________________________________________________________ 
    
     Table 3 were prepared in the same manner as the catalysts identified with Nos. 11 through 15. Note that the loading amount of Co was identical with that of the catalysts identified with No. 13. 
     Except that each of the honeycomb supports having the alumina coating layer was immersed into a cobalt nitrate aqueous solution, and that, after taking each of them out of the solution, blowing each of them to blow away water droplets in excess and drying each of them, each of them was calcinated at 500° C. for 3 hours, comparative catalysts identified with Nos. 19 through 21 which were free from the formation of Co-aluminate were produced in the same manner as the catalysts identified with Nos. 11 through 15. Note that the loading amounts of Co and Ba were identical with those of the catalysts identified with Nos. 11 through 13, respectively. 
     The comparative catalysts identified with Nos. 19 through 21 relate to the catalysts which are employed to carry out the process according to the first aspect of the present invention. 
     Examination for Purifying Performance 
     Each of catalysts Nos. 11 through 18 as well as comparative catalysts Nos. 19 through 21 was disposed in an exhaust line of an actual vehicle equipped with a lean burn engine. The lean burn engine had a displacement of 1.6 liters. The vehicle was driven in the urban area running mode, for instance, it was alternately driven in the 10-mode for a certain period of time and then the 15-mode for another certain period of time, thereby examining the catalysts for the conversion of HC, CO and NO x . The results of the examination are also summarized in the columns designated at &#34;Initial Conversion&#34; in Table 3. 
     After the aforementioned examination, each of catalysts Nos. 11 through 18 as well as comparative catalysts Nos. 19 through 21 was subjected to a bench test on durability which utilized the same type of engine as above. Namely, each of them was disposed in an exhaust line of the engine, and then the engine was run for 50 hours at an air-fuel ratio A/F of 18 while adjusting the temperature of the exhaust gas introduced into each of the catalysts at 800° C. After this bench test was over, each of them was again disposed in the exhaust line of the actual vehicle. The vehicle was driven in the same manner as described above, thereby examining the catalysts for the conversion of HC, CO and NO x  after a durability test. The results of the examination are also summarized in the columns designated at &#34;Conversion after Durability Test&#34; in Table 3. Note that, in the bench test on durability, a fuel was used which contained sulfur in an amount of 30 ppm. 
     It is apparent from Table 3 that the comparative exhaust-gases-purifying process employing comparative catalysts Nos. 19 through 21 was superior to the exhaust-gases-purifying process of the Second Preferred Embodiment employing the catalysts Nos. 11 through 18 in terms of the initial NO x  conversion, but that the comparative exhaust-gases-purifying process exhibited the conversions deteriorated to large extent after the durability test. It is believed that, in comparative catalysts Nos. 19 through 21, the Co is caused to react with the alumina by the heat (800° C.) developed during the durability test, thereby producing compounds which degrade the Co in terms of its activity. 
     On the other hand, in the exhaust-gases-purifying process of the Second Preferred Embodiment employing the catalysts Nos. 11 through 18, the NO x  conversion degraded less even after the durability test. In particular, in the exhaust-gases-purifying process of the Second Preferred Embodiment employing the catalysts whose Co dissolving amount fell in a range of from 0.3 to 0.7 moles with respect to 1 liter of the honeycomb support, the NO x  conversion degraded least. Moreover, in the exhaust-gases-purifying process of the Second Preferred Embodiment, when the Co was loaded on the catalysts in an amount larger than that of the comparative catalysts Nos. 19 through 21, the initial NO x  conversion was equivalent to that exhibited by the comparative exhaust-gases-purifying process employing the comparative catalysts. 
     Thus, even after the durability test, the NO x  conversion can be apparently improved by including the Co in the form of Co-aluminate. This advantage is believed to be effected by inhibiting the NO x  adsorbent from being poisoned and deteriorated by the sulfur. 
     Having now fully described the present invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit or scope of the present invention as set forth herein including the appended claims.