Process for catalytically reducing nitrogen oxides from exhaust gas

Abstract: A catalyst for reducing nitrogen oxides from an exhaust gas containing the same in an oxygen rich atmosphere comprising (i) a zeolite having a molar ratio of SiO₂/Al₂O₃ of at least 10, (ii) copper, and (iii) a rare earth ion, an alkaline earth metal and/or a valence variable metal.

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 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. EP-A 0 362 966, which represents a document according to Art. 54(3) and (4) EPC, describes a catalyst comprising a zeolite of a specific formula and containing an alkali or alkaline earth metal and at least one element selected from the group of Ru, Rh, Pd, Ag, Pt, Au, Co, Cu, Cr and Ni. The catalyst is used in a process wherein waste gases containing nitrogen oxides are denitrized by putting the gases into contact with the catalysts in the absence of reducing agents. The known catalyst uses zeolites of the A-type or X-type; this type of zeolite shows a value for the ratio of SiO₂/Al₂O₃ of about 2 to 3. EP-A 0 365 308, which also represents a document according to Art. 54(3) and (4) EPC, describes an exhaust gas purifying catalyst and a process for purifying exhaust gases wherein a catalyst is used comprising copper and/or cobalt supported by zirconium oxide or titanium oxide as a first component in combination with a copper-substituted type zeolite as a second component. EP-A 0 373 665, which furthermore represents a document according to Art. 54(3) and (4) EPC, discloses a method for exhaust gas cleaning comprising bringing the exhaust gas into contact with a crystalline aluminosilicate having a primary particle size of not less than 0.5 µm, containing ions of at least one metal of the group Ib and/or the group VIII of the periodic table in an amount ranging from 0.2 to 0.8 equivalent relative to one gram of aluminium and having a mole ratio of SiO₂/Al₂O₃ of not less than 20. Elements belonging to the group Ib are Cu, Ag, Au and elements belonging to the group VIII of the periodic table are - for example Fe, Co, Ni. Accordingly, the objects of the present invention are to eliminate the above-mentioned disadvantages of the prior art and to provide a method using a catalyst for reducing NOx, as well as CO and HC, which method 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. This object is achieved by the method according to claim 1. Other objects and advantages of the present invention will be apparent from the following description. In accordance with the present invention, the present method uses a catalyst for reducing nitrogen oxides, carbon monoxide, and hydrocarbons from an exhaust gas containing the same in an oxygen rich atmosphere said catalyst comprising (i) a zeolite having a mole ratio of SiO₂/Al₂O₃ of at least 10 and (ii) copper and at least one kind of alkaline earth metal loaded thereon, wherein the content of copper and of the alkaline earth metal is defined as given in claim 1. The present invention will be better understood from the description set forth below with reference to the accompanying drawing of Figure 1, which is a graph showing the relationships of the NOx conversion versus the temperature of the catalyst No. 2-2 of the Example and Comparative Example catalyst No. C2-1. 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 usable 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 used in the method 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 1nm (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. The catalyst for purifying exhausted gas catalyst according to the method of the present invention is a catalyst for reducing nitrogen oxides from an exhausted gas in an oxygen rich atmosphere in the presence of organic compounds or hydrocarbons, and comprises Cu and an alkaline earth metal, contained in a specific zeolite support, and thus has a better NOx reducing ability than the zeolite catalyst having only Cu contained therein under an oxygen rich atmosphere. Cu has an ability to selectively adsorb NOx, and has a better NOx adsorbing ability than other elements. On the other hand, an alkaline earth metal itself adsorbs NO and NO₂ and has a catalytic activity, whereby a superior effect can be exhibited by both Cu and an alkaline earth metal in combination. The reaction whereby the catalyst according to the present invention exhibits a superior effect has not been clarified, but it is considered that NO is adsorbed by Cu, and NO and NO₂ by the alkaline earth metal, and the NO and the like are immediately subjected to a catalytic reaction with the organic compounds (or hydrocarbons) in the exhausted gas, to be thereby reduced to harmless N₂. The reaction at this time is considered to be as follows: uHC + vNO → wH₂O + yCO₂ + zN₂ (1) Also, this catalyst exhibits a very high durability even at a high temperature of 800°C. Usually, Cu ions are reduced at a temperature of 600 to 800°C and disadvantageously migrated and agglomerated on the zeolite, to thereby lower the durability thereof. In the present method, by permitting alkaline earth metal ions, which are hard to be reduced to metals even at a high temperature and are moved only with difficulty, to exist between Cu ions, the movement of Cu when reduced to metal is prevented, and thus any lowering of the catalytic activity is prevented and the durability is improved. Also, the strong acid site, which is the ion-exchangeable site in the zeolite, becomes a cause of a coke formation in which a large number of graphites formed by a decomposition of organic compounds are bonded together. This may also lead to micro-pore clogging by the coke, and thus to the eventual destruction of the structure of the zeolite. Since the alkaline earth metal extinguishes superfluous strong acid sites participating in the coke formation, among the adsorption sites of hydrocarbons, it can prevent coke formation and thereby, prevent a deterioration of the catalyst. Also, the catalyst according to the present invention has an ability to reduce nitrogen oxides at 200°C or lower. Nitrogen oxides exist as NO₂ at 200°C or lower, and neither Cu nor the alkaline earth metal have the ability to convert reduce NO₂ to N₂ , but since the alkaline earth metal can adsorb NO₂ at 200°C or lower, NO₂ can be reduced from the exhausted gas. The adsorbed NO₂ will be spontaneously decomposed into NO and O₂ at 200°C or higher, and thus NO can be reduced to N₂ by the catalytic ability of Cu and the alkaline earth metal. The degree of Cu loaded is 5 to 80% by atom, based on Al atoms in the zeolite. When smaller than 5%, a satisfactory catalyst effect can not be obtained. The catalytic performance will be improved when the degree of Cu contained is increased, but when 80% or more of Cu is contained, the Cu will be susceptible to deterioration due to migration and agglomeration, and the alkaline earth metal will be difficult to load. At least one kind of the alkaline earth metal is loaded, and the alkaline earth metal is preferably magnesium (Mg), calcium (Ca), strontium (Sr), or barium (Ba). The content is 0.1 to 10% by weight based on the zeolite. The required effect can be exhibited from 0.1% by weight, but to obtain a desired effect, 0.3% by weight or more is preferable. When the content exceeds 10%, however, the acid sites necessary for the reaction will be reduced, and thus the reaction between NO and hydrocarbons will proceed only with difficulty. When the content of the alkaline earth metal carried is increased, generally the optimum temperature of which NOx can be reduced is shifted to a higher temperature, and therefore, it is necessary to increase or decrease the content depending on the catalyst use conditions. The Cu is loaded according to the ion exchange method. Namely, an ion exchange is effected through an exchange of H⁺ or NH + / 4 introduced at ion exchange sites having a negative charge in the lattices of the zeolite, with Cu ions. Cu ions are employed in the form of a soluble salt, and as the soluble salt, an acetate, nitrate, formate or copper ammine complex can be employed. The Cu ion exchange is facilitated if the ion exchange is effected in a solution made slightly basic by an addition of ammonia. The pH of the solution is preferably from 9 to 12, because the ratio of cations of Cu relative to the total cations is high. The alkaline earth metal may be loaded by either the ion exchange method or the impregnation method, and for both the ion exchange method and the impregnation method, an acetate or nitrate of the alkaline earth metal may be employed. An example of the ion exchange method, which is the same as in the case of Cu, comprises the ion exchange step of immersing a zeolite in the solution of the above salt for one day and night, the drying step of heating the zeolite at a temperature of 100 to 120°C for about 10 hours, and the calcination step of holding the product at a temperature of 300 to 700°C for several hours. An example of the impregnation method comprises immersing the zeolite in an aqueous solution of the above salt for 1 to 2 hours, then drying in atmosphere to make the salt loaded thereon. The adhesive force is stronger when loaded according to the ion exchange method. The order in which Cu and the alkaline earth metal are introduced is not limited, but since Ba and Sr can be easily substituted with the loaded Cu, preferably they are introduced prior to the Cu. The catalyst having Cu and the alkaline earth metal contained therein may have any shape or structure, such as powder, pellets or honeycomb shape, etc. Also, a binder such as alumina sol or silica sol may be added to a powdery catalyst, and the mixture molded to a desired shape, or formed into a slurry with an addition of water, which may be in turn coated onto a refractory substrate of alumina, etc., having a honeycomb shape. The catalyst used in accordance with the present invention reduces an exhausted gas through a reaction of NOx in the discharged gas with organic compounds, i.e., noncombusted hydrocarbons or oxygen containing organic compounds formed by a partial oxidation. Preferably, these hydrocarbons are those remaining in the exhausted gas, but when the amount thereof is lower than that necessary for carrying out the reaction, preferably the hydrocarbons are added to the exhausted gas from an external source. The catalyst used in accordance with the method of the present invention purifies an exhausted gas through the reaction with NOx contained therein. As organic compounds, hydrocarbons such as unburned olefins and paraffins and oxygen-containing compounds formed by a partial oxidation can be employed. Among these compounds, olefins and oxygen-containing organic compounds having functional groups (e.g., double bond) and having a strong adsorption force are preferred. The hydrocarbons remaining in the exhausted gas may be used, but when present in an amount less than that necessary for carrying out the reaction, it is preferable to add hydrocarbons to the exhausted gas from an external source. EXAMPLES The present invention will now be further illustrated by, but is by no means limited to, the following Examples. Example 2-1 Catalysts were prepared, and the conversion activity thereof for NO was evaluated by using a model gas under an oxygen-rich lean condition. The same activity evaluation was performed for the Comparative catalyst. Preparation of Catalysts of the Present Example (No. 2-1 to 2-9) and Comparative Catalyst (No. C2-1) An H type ZSM-5 (SiO₂/Al₂O₃=40) powder zeolite was dipped overnight in respective aqueous solutions of acetates of Mg, Ca, Sr, and Ba (0.2% by weight as the metal amount) to effect an ion exchange, then washed with water and dried at 100°C for 3 hours, so that Mg, Ca, Sr and Ba were loaded thereon, respectively. Next, according to the same method as used for the H type ZSM-5, Ca was loaded, by an ion exchange using calcium acetate, on an H type Y zeolite (SiO₂/Al₂O₃=15) and H type mordenite (SiO₂/Al₂O₃=30) powder. Each sample was dipped in an aqueous Cu ammine complex solution (containing 0.2% by weight of Cu atoms), then subjected to an overnight ion exchange, and further, dried at 100°C for 3 hours to obtain the catalysts No. 2-1 to 2-6 of the Example having Cu and alkaline earth metals loaded thereon, as shown in Table 2-1. Also, using an H type ZSM-5, a Comparative catalyst No. C2-1 having only Cu loaded thereon was prepared according to the same operation as described above. The amounts of Cu loaded on these catalysts are shown in Table 2-1. Further, the powders of NH₄ type ZSM-5, NH₄ type Y zeolite, and NH₄ type mordenite were dipped in an aqueous Cu ammine complex solution (containing 0.2% by weight of Cu atoms), respectively, subjected to an overnight ion exchange, washed with dil. ammonia water, and dried at 100°C for 3 hours. Each of these powders was then dipped in a calcium nitrate solution (containing 1% by weight of Ca atoms) for water absorption so that they loaded Ca by impregnation, whereby the catalysts of the Example (No. 2-7 to 2-9) were obtained. The amounts of Cu and Ca loaded on these catalysts are shown in Table 2-1. Catalyst Activity Evaluation The catalysts No. 2-1 to 2-9 of the present Example and Comparative catalyst No. C2-1 were formed into pellets and were maintained in the model gas shown in Table 2-2, at 800°C for 5 hours. Then, the NOx conversion efficiency in the model gas shown in Table 2-2 was measured at 400°C under a steady state, after allowing to stand at 400°C for several minutes. The results are snown in Table 2-3. As can be seen from Table 2-3, the catalysts No. 2-1 to 2-9 of the present Example have a much greater durability than the Comparative catalyst No. C2-1. Powders of the catalyst No. 2-2 of the present Example and Comparative catalyst No. C2-1 were then compression molded into pellets about 3 mm⊘, and the pellets were subjected to a measurement of NOx conversion characteristics from room temperature to 600°C, using the model gas under an oxygen rich lean condition (air to fuel ratio A/F = 18 ) as shown in Table 2-2. The space velocity was 30,000 hr⁻¹ and the temperature elevation rate 10°C/min. The results are shown in Fig. 1. It can be seen from the Figure that the catalyst of the present Example has a superior NOx conversion characteristic at low temperatures of 250°C or lower and at 400°C or higher. The conversion efficiencies of CO and C₃H₆ are also shown in Table 2-3. Model gas composition (Vol%) CO 0.1 H₂ 0.033 C₃H₆ 0.04 NO 0.05 NO₂ 0.017 O₂ 4.0 CO₂ 10.0 H₂O 3.0 N₂ Balance Catalyst No. Conversion efficiency NOx (%) CO (%) C₃H₆ (%) 2-1 40 80 85 2-2 52 84 87 2-3 44 83 86 2-4 45 81 82 2-5 33 73 74 2-6 44 79 82 2-7 54 83 88 2-8 43 77 85 2-9 35 76 79 C2-1 25 41 44 Example 2-2 The durability of the catalyst used according to the present invention while driving an automobile in an urban district was evaluated. The each powder (500 g) of the catalyst No. 2-2 and No. C2-1 shown in Table 2-1 was mixed, while stirring, with 700 g of a silica sol (10% by weight of SiO₂) and 100 g of pure water, the mixture was adjusted to a pH of 10 to 11 with ammonia water, to obtain a slurry for coating, and the slurry was coated onto a 0.7 liter cordierite honeycomb carrier at 120 g/liter. The carrier coated with the catalyst No. 2-2 is the catalyst No. 2-10 catalyst of the present Example, and the carrier coated with No. C2-1 is the Comparative Example No. C2-2. The initial catalytic activity and durability of these catalysts were evaluated under actual driving conditions. Durability Test Conditions The catalyst No. 2-10 of the present Example and the Comparative catalyst No. C2-2 were each mounted in an engine gas exhausting system of a car having a weight of 1 ton and a 1600 cc lean burning engine, and the car was run under conditions simulating a running of the vehicle in urban district, wherein the inlet gas temperature was about 800°C at a maximum after 1000 km and 30,000 km. NOx conversion efficiency measurement The NOx conversion efficiency was measured at an average air to fuel ratio of 22 and an inlet gas temperature of 400°C, for the above-mentioned engine. The results are shown in Table 2-4. Catalyst No. NOx conversion efficiency (%) Before durability test After 1,000 km running After 30,000 km running 2-10 55 53 55 C2-2 59 59 20 Although the H type ZSM-5 having only Cu contained therein is durable to some extent, but it can be appreciated that the H type ZSM-5 having Cu and Ca contained in combination therein has a superior durability thereto.

Claims: A method of purifying an exhaust gas containing a stoichiometrically excess amount of oxygen, said method comprising the step of bringing a catalyst into contact with the exhaust gas in the presence of an organic compound 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) copper ion and at least one kind of alkaline earth metal loaded thereon, wherein the amount of the copper ion is 5 % to 80 %, in terms of atomic ratio, based upon the atom of Al in the zeolite and wherein the amount of the alkaline earth metal is 0.1 % to 10 % by weight based upon the weight of the zeolite. A method according to claim 1, wherein said alkaline earth metal is at least one kind of metal selected from the group consisting of magnesium, calcium, strontium and barium. A method according to claim 1, further comprising at least one kind of rare earth ion loaded on the zeolite. A method according to claim 3, wherein the amounts of the copper ion, the alkaline earth metal, and the rare earth ion are 5 % to 80 %, in terms of atomic ratio, based upon the aluminum atom in the zeolite, 0.05 % to 2 % by weight based upon the weight of the zeolite, and 0.1 % to 10 % by weight based upon the weight of the zeolite, respectively.

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
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: B01J29
IPC codes: B01J29, B01D53