Process for reducing nitrogen oxides from exhaust gas

Abstract: A method for purifying an exhaust gas containing an excess amount of oxygen is described, wherein a catalyst is brought into contact with the exhaust gas to remove nitrogen oxides, carbon monoxide and hydrocarbons from the exhaust gas, said catalyst comprises: (i) a zeolite having a molar ratio of SiO₂/Al₂O₃ of at least 10; and (ii) at least one copper ion and at least one kind of rare earth ion loaded thereon.

Description: The present invention relates to a method for reducing nitrogen oxides from a gas containing the same, such as gases exhausted from internal combustion engines (e.g., automobile engines) and those exhausted from industrial plants (e.g., nitric acid production plants). More specifically it relates to a method for reducing nitrogen oxides as well as carbon monoxide and hydrocarbons from the above-mentioned exhaust gases in an oxygen rich atmosphere and a method of reducing nitrogen oxides as well as carbon monoxide and hydrocarbons from the above-mentioned exhaust gases in an oxygen rich atmosphere. The term "an oxygen rich atmosphere" used herein means an atmosphere having an oxygen content larger (i.e. a stoichiometric excess) than that of a theoretical air-fuel ratio. Over many years the harmful components of nitrogen oxides (NOx) contained in exhausted gases have been exhausted from internal combustion engines such as those used by, for example, automobiles, and from, for example, nitric acid production plants, to cause atmospheric pollution, and accordingly, various methods of reducing nitrogen oxides from such exhausted gases have been investigated. Further, a diluted combustion has been considered, to lower the fuel costs of automobiles. In this case, the air to fuel ratio is made an oxygen rich atmosphere on the lean side, whereby drawbacks have occurred in that the three-way catalyst having noble metals (e.g., Pt, Rh, Pd), loaded on a carrier such as Al₂O₃ , as used in the prior art, can not eliminate NOx emissions, although HC and CO, among the harmful components in the exhausted gas, are removed. A known catalyst which solves this problem is a copper (Cu)-containing zeolite (Japanese Unexamined Patent Publication (Kokai) No. 63-283727). The basic principle for removing NOx in an oxygen rich atmosphere with a Cu-containing zeolite catalyst resides in adsorbing NOx with the Cu and effecting a catalytic reaction of the adsorbed NOx with the reducible uncombusted hydrocarbons contained in the exhausted gas, to reduce the NOx to N₂. This catalyst exhibits a NOx conversion ability at 200°C or higher, but has the problems described below. The Cu-containing zeolite catalyst has an excellent initial catalytic activity, since Cu has a high NOx adsorption ability, but a problem arises therewith of durability, particularly durability at high temperatures. Accordingly, the development of a catalyst having a high catalytic activity even after prolonged usage is required. The cause of the inferior durability of this catalyst is the migration and agglomeration of the copper in the zeolite at a temperature of 600°C or higher, whereby the catalyst activity is lowered, and further, because the stability of the zeolite structure is lowered by containing Cu therein, and the structure thereof is destroyed after prolonged usage. Also, since the above-mentioned catalyst cannot reduce NOx at 200°C or lower, there is a problem in that NOx cannot be reduced at 200°C or lower. This is because NOx exists as NO₂ under an oxygen rich condition of 200°C or lower, but the Cu-containing zeolite cannot convert NO₂ to N₂. The US-A-4 297 328 describes the use of Cu-containing zeolite catalysts having a SiO₂/Al₂O₃ ratio of at least 20 for purifying exhaust gases containing nitrogen oxides, carbon monoxide and hydrocarbons. It is said that the zeolites may be exchanged with ammonium, or other cations, including metal ions, hydrogen ions, rare earth ions and mixtures thereof. Accordingly, the objects of the present invention are to eliminate the above-mentioned disadvantages of the prior art and to provide a novel method for reducing NOx, as well as CO and HC, in which method the catalyst has a higher conversion ability than the catalyst of the prior art in a broad temperature region of 800°C or lower under an oxygen rich atmosphere, without using a reducing agent such as ammonia, and has a good durability (or service life) with no significant lowering of the catalytic activity thereof even when used for a long time. Other objects and advantages of the present invention will be apparent from the following description. In accordance with the present invention, there is provided a method for purifying an exhaust gas containing a stoichiometric excess amount of oxygen, wherein a catalyst is brought into contact with the exhaust gas to remove nitrogen oxides, carbon monoxide and hydrocarbons from the exhaust gas, said catalyst comprising: (i) a zeolite having a molar ratio of SiO₂/Al₂O₃ of at least 10; and (ii) at least one copper ion in an amount of 0.01 to 1 in terms of a Cu/Al atomic ratio, and at least one kind of rare earth ion selected form the group consisting of La, Ce, Pr, Nd, Pm and Sm in an amount of at least 0.01 in terms of a rare earth ion/Al atomic ratio. The zeolite usable in the present invention has a general composition of: xM 2 / nO·Al₂O₃·ySiO₂·zH₂O wherein n is a valence of a cation M, x is a number of from 0.8 to 2, y is a number of 2 or more, and z is a number of 0 or more. That is, the zeolite is constituted of a tetrahedral network structure of SiO₂ and Al₂O₃ , and the individual tetrahedral structures are bonded at the corners thereof by a crosslinking with oxygen, to thereby form a network structure through which channels and cages are extended, and at the ion exchangeable site (strong acid site) having a negative lattice charge, an exchangeable cation (H⁺, Na⁺, etc.) is introduced. The zeolite used in the present invention has a molar ratio of SiO₂/Al₂O₃ of 10 or more, preferably 10 to 200. When the SiO₂/Al₂O₃ mole ratio is less than 10, due to a low heat resistance and durability of the zeolite itself, a required heat resistance and durability of the catalyst cannot be obtained. The zeolite constituting the catalyst of the present invention may be a natural or a synthetic product, and the methods of preparation of these zeolites are not particularly limited. Nevertheless, representative examples of zeolites to be used include mordenite, ferrierite, ZSM-5, ZSM-11, ZSM-12, and ZSM-20. Also, zeolites II such as Y type zeolite and L type zeolite subjected to dealumination may be employed. Also, these zeolites can be used as such or treated with an ammonium salt and a mineral acid to be ion exchanged to form NH + / 4 or H⁺ before use as the catalyst of the present invention. Also, preferably the zeolite is one having NH + / 4 and H⁺ readily exchangeable with Cu ion site. Also, the micro-pores on the zeolite surface are preferably as small as 1.0 nm (10 Å) or less. By making the size of the micro-pores such that polycyclic aromatic hydrocarbons cannot enter therein, coke will be formed with difficulty, and the structural destruction and lowering of the catalytic activity due to the clogging of micro-pore can be prevented. As the method of incorporating copper ion and a rare earth ion in the above-mentioned zeolite, the ion exchange method is included. The method is not particularly limited, but generally practiced ion exchange methods can be employed. A representative example is the method in which an ion exchange is effected by using a mixed aqueous solution containing copper and a rare earth ion, or an ion exchange of the rare earth ion may be effected after an ion exchange of copper ions. Alternatively, an ion exchange of the copper ions may be effected after an ion exchange of a rare earth ion. The concentrations of copper and a rare earth ion during the ion exchange can be set as desired, depending on the intended degree of ion exchange. In the above description, during the copper ion exchange, to increase the copper ion content, ammonia may be added to adjust the pH. Since the copper ions existing at the ion-exchangeable sites are active sites, preferably the copper is ion exchanged at those ion-exchangeable sites. For the rare earth ion, if the pH is increased by an addition of ammonia, the rare earth ion will be precipitated as hydroxide, and it becomes difficult to effect the ion exchange, and therefore, preferably ammonia is not added. Also, the rare earth ion is preferably ion-exchanged at the ion-exchangeable site, but the effect thereof can be also exhibited when loaded as an oxide on the zeolite, by a calcination of the hydroxide precipitated on the zeolite surface. The sample after ion-exchange is subjected to solid-liquid separation, washing and drying before use as the catalyst, and if necessary, it can be calcined before use. The degree of ion-exchange of copper is within the range of from 0.01 to 1, preferably from 0.1 to 0.6 as represented by the Cu/Al atomic ratio. When the Cu/Al atomic ratio is less than 0.01, copper ions which are active sites become smaller, and thus a required catalytic activity cannot be obtained. Conversely when the ratio exceeds 1, excess copper will exist in an agglomerated state on the zeolite surface, whereby an adverse influence may be exerted on the catalytic activity, heat resistance, and durability. The degree of ion-exchange amount of the rare earth ion is 0.01 or more, preferably 0.01 to 2, in terms of (rare earth ion)/Al atomic ratio. When the (rare earth ion)/Al atomic ratio is less than 0.01, the effect of the co-presence of the rare earth ion is small, and thus a satisfactory catalyst performance and durability cannot be obtained. The SiO₂/Al₂O₃ molar ratio of the exhausted gas purification catalyst of the present invention is not substantially different from the SiO₂/Al₂O₃ molar ratio of the zeolite substrate employed. Also, the crystal-structure of the exhausted gas cleaning catalyst will be the same before and after the ion exchange. The exhaust gas purification catalyst of the present invention can be also used by mixing and molding together with a binder such as a clay mineral, and the zeolite can be previously molded and copper and a rare earth ion incorporated by an ion exchange in the molded product. Examples of the binder used during the molding of zeolite include clay minerals such as kaolin, atapalgite, montmorillonite, bentonite, allophen, and sepiorite. Alternatively, a binderless zeolite obtained by a direct synthesis of a molding may be employed. Further, a zeolite can be also used by wash coating same onto a honeycomb-shaped substrate made of cordierite or a metal. Nitrogen oxides, carbon monoxide and hydrocarbons in an oxygen rich exhausted gas can be reduced by bringing the exhausted gas cleaning catalyst of the present invention into contact with the oxygen rich exhausted gas containing nitrogen oxides, carbon monoxide and hydrocarbons. The oxygen rich exhausted gas to be treated by the present invention refers to an exhausted gas containing a larger amount of oxygen than is necessary for a complete oxidation of carbon monoxide, hydrocarbons and hydrogen contained in the exhausted gas (i.e. is in stoichiometric excess), and specific examples of such an exhausted gas include gases exhausted from internal combustion engines such as those used in automobiles, etc., particularly exhausted gases having a high air to fuel ratio (i.e., lean region). The above-described exhausted gas catalyst can also exhibit a catalytic performance equal to that of the zeolite catalyst of the prior art having copper loaded thereon, when applied to an exhausted gas containing carbon monoxide, hydrocarbons and hydrogen which is not in oxygen rich. As described above, in the exhausted gas under a non-co-presence of a reducing agent, nitrogen oxides are decomposed into oxygen and nitrogen through a redox reaction of the copper ions exchanged into the zeolite. On the other hand, in the gas exhausted from internal combustion engines where oxygen is co-present, nitrogen oxides are reduced on the base metals in the zeolite with hydrocarbons and carbon monoxide existing in minute amounts in the exhausted gas, and oxygen-containing organic compounds formed by a partial oxidation. In this case, at the base metal, copper ions exhibit the best catalyst performance. Copper ions, not at the ion-exchangeable sites but when loaded as oxides on the zeolite, or when the copper is agglomerated, have a low adsorption capacity of nitrogen oxides, and when exposed to high temperatures, tend to promote a release of the Cu ions loaded at the ion-exchangeable sites, and therefore, preferably are loaded at the ion-exchangeable sites, to ensure a required catalytic performance and durability. The catalyst of the present invention has copper ions and a rare earth ion co-present on a zeolite as described above, and the effects obtained by the co-presence of copper ions and a rare earth ion may be considered to be as follows. 1) The heat resistance and durability of the zeolite itself are improved. 2) By permitting rare earth ions to be interposed between the copper ions in the zeolite, the migration and agglomeration of copper ions are prevented. 3) Due to the weak acid site of rare earth element ions, the formation of coke is prevented, and various hydrocarbons contained in the exhausted gas are converted to lower olefins and oxygen-containing compounds. The copper ions ion-exchanged into the zeolite will be released from the ion-exchangeable sites when treated at a high temperature, to be diffused and agglomerated within micro-pores or toward the zeolite surface, whereby the reaction between the nitrogen oxides and the reducing components in the exhausted gas will be hindered. In contrast, by permitting rare earth ions to be co-present with copper ions, the migration and agglomeration of copper ions can be prevented to thereby improve the durability of the catalyst. Also, various hydrocarbons are contained in the gases exhausted from internal combustion engines. The hydrocarbons effective for reducing nitrogen oxides are lower olefins and oxygen containing organic compounds; paraffins and aromatic compounds have a lower reducing ability of nitrogen oxides. In such a case, in the catalyst of the present invention the rare earth ions form weak acid sites in the zeolite, thereby providing the effect that hydrocarbons having a lower reactivity, such as paraffins, can be converted to lower olefins and oxygen-containing organic compounds having a higher reactivity. Accordingly, due to the reaction of the converted lower olefins and nitrogen oxides on the copper ions, hydrocarbons and nitrogen oxides can be removed at the same time, to thereby improve the catalytic activity. Also, the weak acid sites formed by the rare earth ions suppress the formation of coke, to thereby improve the durability of the catalyst. The exhausted gas reducing catalyst used in the present invention enables the removal of nitrogen oxides, carbon monoxides, and hydrocarbons, at the same time, from an oxygen rich exhausted gas, and exhibits a very high heat resistance and durability. Therefore, by bringing the catalyst of the present invention into contact with an exhausted gas, even under an oxygen rich state, nitrogen oxides, carbon monoxide and hydrocarbons in the gas can be reduced. EXAMPLES The present invention will now be further illustrated by, but is by no means limited to, the following Examples. Example 1-1 An aqueous sodium silicate solution (SiO₂; 250 g/liter, Na₂O; 82 g/liter, Al₂O₃; 2.75 g/liter) and an aqueous aluminum sulfate solution (Al₂O₃; 8.75 g/liter, SO₄; 365 g/liter) were continuously fed, respectively, at flow rates of 1.5 liter/hr and 0.5 liter/hr, while stirring into an overflow type reactor (actual volume, one liter). The apparent residence time of the reaction slurry was 30 minutes, the reaction temperature was 30 to 32°C, and the pH of the slurry was from 6.3 to 6.6. The exhausted slurry was separated into a solid and a liquid, and then washed. The obtained wet cake had the following composition, as represented by the molar ratio of oxides obtained by a chemical analysis: 1.6 Na₂O·Al₂O₃·79 SiO₂·460 H₂O The thus obtained wet cake (2.89 kg) and 6.11 kg of an aqueous 3.5 wt.% sodium hydroxide NaOH solution were charged into an autoclave and heated at 165°C for 72 hours to synthesize a zeolite similar to ZSM-5. The zeolite had the following chemical composition, as represented by the molar ratio of oxides in an anhydrous base: 1.1 Na₂O·Al₂O₃·43 SiO₂ (SiO₂/Al₂O₃=43). The zeolite (1 kg) was added to an aqueous mixed solution of 0.1 mol/liter cerium nitrate and 0.1 mol/liter of copper acetate to one-fold of Cu atoms and 5-fold of Ce atoms relative to Al atoms in the zeolite, and the mixture was stirred at room temperature for 20 hours to effect an ion exchange. After this operation was repeated three times, the mixture was washed and dried to prepare a catalyst No. 1-1. Also, using La and Nd in place of Ce, catalyst Nos. 1-2 and 1-3 were prepared in the same manner as described above. The results of chemical analyses of the obtained catalysts are shown in Table 1-1. Catalyst M Cu/Al M/Al Catalyst 1-1 Ce 0.32 0.20 Catalyst 1-2 La 0.38 0.15 Catalyst 1-3 Nd 0.38 0.13 Example 1-2 A mordenite type zeolite was synthesized according to the method of Example 5 of Japanese Unexamined Patent Publication (Kokai) No. 59-735. The zeolite had the following chemical composition, as represented by the molar ratio of oxides in an anhydrous base: 1.1 Na₂O·Al₂O₃·17.5 SiO₂ (SiO₂/Al₂O₃=17.5) The zeolite (100 g) was added to an aqueous mixed solution of 0.1 mol/liter cerium nitrate and 0.1 mol/liter of copper acetate to one-fold of Cu atoms and 5-fold of Ce atoms relative to Al atoms in the zeolite, and the mixture was stirred at room temperature for 20 hours to effect an ion exchange. After this operation was repeated three times, the mixture was washed and dried to prepare a catalyst No. 1-4. The results of chemical analyses of the obtained catalyst are shown in Table 1-2. Catalyst Cu/Al Ce/Al Catalyst 1-4 0.31 0.24 Catalyst 1-5 0.33 0.19 Example 1-3 A ferrierite type zeolite was synthesized according to the method of Example 1 of Japanese Unexamined Patent Publication (Kokai) No. 60-141617. The zeolite had the following chemical composition, as represented by the molar ratio of oxides in an anhydrous base: 0.3 Na₂O·0.7 K₂O·Al₂O₃·17 SiO₂ (SiO₂/Al₂O₃=17) The zeolite (100 g) was added to an aqueous mixed solution of 0.1 mol/liter cerium nitrate and 0.1 mol/liter of copper acetate to one-fold of Cu atoms and 5-fold of Ce atoms relative to Al atoms in the zeolite, and the mixture was stirred at room temperature for 20 hours to effect an ion exchange. After this operation was repeated three times, the mixture was washed and dried to prepare a catalyst No. 1-5. The results of chemical analyses of the obtained catalyst are also shown in Table 1-2. Example 1-4 The same zeolite as used in Example 1-1 (200 g) was added to an aqueous solution of 0.1 mol/liter copper acetate to 1-fold Cu atoms relative to the Al atoms in the zeolite, and the pH was adjusted to 11 by an addition of 2.5% ammonia water. After stirring at room temperature for 20 hours, the mixture was washed and dried to prepare a Cu/zeolite, and as a result of a chemical analysis thereof, the Cu/Al atomic ratio was found to be 0.48. A 100 g amount of the Cu/zeolite was added to an aqueous 0.1 mol/liter aqueous cerium acetate solution to 0.5-fold Ce atoms relative to the Al atoms in the zeolite, and the mixture was stirred at room temperature for 20 hours to effect an ion exchange. The product was then subjected to solid-liquid separation and drying to prepare a catalyst No. 1-6. As a result of a chemical analysis thereof, the Ce/Al atomic ratio was found to be 0.31. Example 1-5 The respective catalysts were press molded and crushed, followed by classification with a 42 to 80 mesh. An amount of each of the particles was filled in an atmospheric pressure fixed-bed flow type reaction tube, and a gas simulating the exhausted gas of a lean burning engine (having the gas composition shown in Table 1-3) was made to flow therein at a space velocity of 30000/hr. Gas composition Vol% CO 0.1 H₂ 0.03 C₃H₆ 0.04 NO 0.06 CO₂ 10 H₂O 3 O₂ 4 N₂ balance While passing the same gas, a pre-treatment was performed at 500°C for 30 minutes, cooling to room temperature under the same gas condition followed by a temperature elevation at 5°C/min from room temperature to 600°C, and the conversion activity was measured. The conversion efficiencies of NO, CO, and C₃H₆ at 400°C are shown in Table 1-4. The conversion efficiencies were determined by measuring each component concentration of the gas after having passed through the catalyst, and calculating same as the conversion efficiencies relative to the gas in Table 1-3. Also, each catalyst was aged at 700°C for 5 hours while flowing the gas of the composition in Table 1-3 at a space velocity of 30000/hr. Then, the purification efficiencies were measured according to the same method as described above, to test the durability. The results are shown in Table 1-4. Catalyst Conversion efficiency (%) NO CO C₃H₆ Catalyst No. 1-1 Fresh catalyst 79 92 95 Aged catalyst 56 85 89 Catalyst No. 1-2 Fresh catalyst 78 91 95 Aged catalyst 58 88 89 Catalyst No. 1-3 Fresh catalyst 81 91 93 Aged catalyst 54 83 85 Catalyst No. 1-4 Fresh catalyst 75 88 90 Aged catalyst 48 82 86 Catalyst No. 1-5 Fresh catalyst 77 90 91 Aged catalyst 53 87 86 Catalyst No. 1-6 Fresh catalyst 81 93 95 Aged catalyst 57 89 88 Comparative catalyst C1-1 Fresh catalyst 53 78 83 Aged catalyst 25 41 43 Comparative Example 1-1 Using the Cu/zeolite (hereinafter referred to as "Comparative catalyst No. C1-1"), the same performance evaluation tests as in Example 1-5 were performed. The results are shown in Table 1-4 above. From the results shown in Table 1-4, when the lower conversion efficiency of the fresh catalyst and the treated catalyst of the Comparative Example are compared, the exhausted gas conversion catalysts of the respective Examples of the present invention have a higher conversion efficiency of the fresh catalyst and aged catalyst, and thus a superior catalyst performance and durability thereof was confirmed. Example 1-6 One hundred (100) parts by weight of the powder of the catalyst No. C1-1 obtained in Example 1-1, 20 parts by weight of a silica sol (solids 10%), and 50 parts by weight of water were mixed and stirred to prepare a slurry. The viscosity of the slurry was found to be 250 cps. The slurry was coated onto the surface of a honeycomb substrate of cordierite with a capacity of 1.7 ℓ and a cell number of about 400, and superfluous slurry was blown off with air. Subsequently, after drying at 100°C for 3 hours, the product was calcined at 500°C for 3 hours to prepare a monolith catalyst containing 120 g of a zeolite layer. The monolith catalyst was mounted on a vessel made of a metal, and then fitted in a lean burn engine exhausting system with an exhausted amount of 1.6 ℓ. The conversion efficiencies of nitrogen oxides (NOx), CO, and hydrocarbons (HC) under simulated urban running conditions were measured. The average air to fuel ratio was 22, at a maximum temperature of about 750°C and a test time of 600 hours. The results are shown in Table 1-5. Conversion efficiency (%) NOx CO C₃H₆ Before test 50 78 84 After 200 hours 47 65 82 After 400 hours 49 63 80 After 600 hours 45 62 79 Comparative Example 1-2 Using the Comparative catalyst (Cu/zeolite) obtained in Example 1-4, a monolith catalyst was prepared in the same manner as in Example 1-6, and the same tests as in Example 1-6 were conducted. The results are shown in Table 1-6. Conversion efficiency (%) NOx CO C₃H₆ Before test 43 63 75 After 200 hours 32 58 71 After 400 hours 21 55 66 After 600 hours 15 53 64 From the results shown in Table 1-6, the catalyst of Comparative Example had a low conversion efficiency before the test and after 200, 400, 600 hours, but the exhausted gas conversion catalysts of the present invention had a higher purification efficiency before the test and after 200, 400 and 600 hours, whereby the superior performance and durability of the catalyst of the present invention was confirmed.

Claims: A method for purifying an exhaust gas containing a stoichiometric excess amount of oxygen, wherein a catalyst is brought into contact with the exhaust gas to remove nitrogen oxides, carbon monoxide and hydrocarbons from the exhaust gas, said catalyst comprising: (i) a zeolite having a molar ratio of SiO₂/Al₂O₃ of at least 10; and (ii) at least one copper ion in an amount of 0.01 to 1 in terms of a Cu/Al atomic ratio, and at least one kind of rare earth ion selected form the group consisting of La, Ce, Pr, Nd, Pm and Sm in an amount of at least 0.01 in terms of a rare earth ion/Al atomic ratio.

Inventors: FUKUI MASAYUKI, IGAWA KAZUSHIGE, ISHIBASHI KAZUNOBU, KAMIYAMA KATUMI, KASAHARA SENSHI, KONDOH SHIROH, MATSUMOTO SHINICHI, NAKANO MASAO, SUZUKI TADASHI, TAKESHIMA SHINICHI, YOKOTA KOJI, FUKUI, MASAYUKI, IGAWA, KAZUSHIGE, ISHIBASHI, KAZUNOBU, KAMIYAMA, KATUMI, KASAHARA, SENSHI, KONDOH, SHIROH, MATSUMOTO, SHINICHI, NAKANO, MASAO, SUZUKI, TADASHI, TAKESHIMA, SHINICHI, YOKOTA, KOJI, Ishibashi, Kazunobu, c/o Toyota Jidosha Kabushiki
Applicants: TOSOH CORP, TOYODA CHUO KENKYUSHO KK, TOYOTA MOTOR CO LTD, KABUSHIKI KAISHA TOYOTA CHUO KENKYUSHO, TOSOH CORPORATION, TOYOTA JIDOSHA KABUSHIKI KAISHA
Application year: 1990
Main classification: B01D53
IPC codes: B01D53, B01J29