Method of purifying exhaust gas

Abstract: A catalyst for and method of purifying an exhaust gas to thereby remove nitrogen oxides, carbon monoxide and hydrocarbons from an oxygen rich exhaust gas containing same, comprising a zeolite having manganese contained therein.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of purifying an exhaust gas to thereby remove nitrogen oxides, carbon monoxide and hydrocarbons contained in an exhaust gas discharged, for example, from the internal combustion engines of automobiles, gas engines in cogeneration system and the like, more particularly, it relates to a method of removing nitrogen oxides contained in an oxygen rich exhaust gas. The term "oxygen rich exhaust gas" used herein is intended to mean an exhaust gas containing oxygen in an amount exceeding the amount of oxygen necessary for completely oxidizing reducing component such as carbon monoxide and hydrocarbons contained in the exhaust gas. Specific examples of such an exhaust gas include exhaust gases discharged, for example, from internal combustion engines of automobiles reducing component such as, particularly exhaust gases produced at a high air/fuel ratio, excess air ratio respectively. (i.e., in the lean burn region). 2. Description of the Related Art The worsening environmental problems have made it important to increase the removal of nitrogen oxides, carbon monoxide, hydrocarbons, etc., emitted from various sources. Namely, nitrogen oxides are discharged in a large amount from various moving sources including internal combustion engines, such as gasoline engines of automobiles, and from fixed sources including boilers in plants and gas engines and gas turbines in cogeneration systems, etc., and thus the purification of such exhaust gases is urgently required, as it is a serious social problem. Currently, a three-way catalyst comprising Pt, Rh, Pd, etc., supported on a carrier material is used as a catalyst for purifying an exhaust gas discharged from internal combustion engines, but since the three-way catalyst cannot remove nitrogen oxides contained in an oxygen-rich exhaust gas, it is used in combination with a system for controlling the air to fuel ratio of an engine (i.e., "air-fuel ratio"). A lean combustion system has been developed, to attempt a lowering of the fuel consumption and a reduction in the amount of carbon dioxide exhaust gas emissions, but an exhaust gas from this lean combustion system contains an excess amount of oxygen, and thus the nitrogen oxides cannot be removed by the above-mentioned three-way catalyst. Nitrogen oxides contained in an oxygen rich exhaust gas are removed by selective catalytic reduction method, wherein ammonia is used. The practical application of this method, however, is limited due to the necessity of increasing the size of the equipment and the hazardous nature of ammonia. Recently, a zeolite catalyst capable of removing nitrogen oxides contained in an oxygen rich exhaust gas without the addition of a special reducing agent such as ammonia has been proposed. For example, JP-A-63283727 (corresponding to EP-A-0286967) and JP-A-1130735 disclose that a zeolite catalyst ion exchanged with a transition metal can remove nitrogen oxides even in an oxygen rich exhaust gas, through the utilization of minor amounts of unburnt hydrocarbons contained therein as a reducing agent. The conventional zeolite catalysts proposed in JP-A-63283727 and JP-A-1130735 have not yet been put to practical use. Further, in the case of internal combustion engines such as gas engines and gas turbines wherein a gaseous fuel is used, the hydrocarbons contained, in a minor amount, in an exhaust gas have mainly one carbon atom, and therefore, the ability of the conventional zeolite catalysts to remove nitrogen oxides contained in this type of exhaust gas is particularly low. SUMMARY OF THE INVENTION Accordingly, the object of the present invention is to eliminate the above-mentioned disadvantages of the prior art and to provide a method for purifying an exhaust gas by the use of a catalyst, by which method nitrogen oxides, carbon monoxide and hydrocarbons are efficiently removed from an oxygen rich exhaust gas containing nitrogen oxides, carbon monoxide and hydrocarbons, in which 80% or more of the hydrocarbons are hydrocarbons with only one carbon atom. Other objects and advantages of the present invention will be apparent from the following description. The object is solved by the method for purifying an exhaust gas according to claim 1. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present inventors have made extensive and intensive studies of the above-mentioned problems, and as a result, have found that a catalyst comprising a zeolite and manganese is capable of efficiently purifying nitrogen oxides, carbon monoxide and hydrocarbons from an oxygen rich exhaust gas, and that the conversion of nitrogen oxides can be further enhanced through the use of such a catalyst and the addition of a hydrocarbon to the exhaust gas, and thus completed the present invention. The present invention will now be described in more detail. The zeolite usable in the present invention is a crystalline aluminosilicate generally having the following composition: xM 2/n O · A1 2 O 3 · ySiO 2 · zH 2 O wherein n is a valency of the cation M, x is 0.8 to 1.2, y is 2 or more and z is 0 (zero) or more, and includes many naturally occurring zeolites and synthetic zeolites known in the art. Although there is no particular limitation on the type of zeolite used in the present invention, those having a silica/alumina mole ratio of 10 or more are preferable; representative examples of the zeolite include ferrierite, Y, mordenite, ZSM-5 and ZSM-11, and among them, ZSM-5 is most preferable. These zeolites may be used as such or after an ion exchange with NH 4 C1, NH 4 NO 3 , (NH 4 ) 2 SO 4 , etc., to form an NH 4 or H type zeolite, and further, may contain cations such as an alkali metal or an alkaline earth metal. The catalyst used for purifying an exhaust gas according to the present invention is characterized by containing manganese. There is no particular limitation on the method of incorporating manganese into the zeolite, and such an incorporation may be conducted by a conventional ion exchange method or impregnation supporting method, etc. There is no particular limitation on the method of carrying out the ion exchange. For example, the ion exchange may be carried out by adding a zeolite to a solution containing the above-mentioned metal ion, followed by stirring the mixture at 20 to 100°C for several hours. Examples of the metal salt used include acetate, nitrate, oxalate and chloride. The method of carrying out the impregnation support also is not particularly limited. For example, the impregnation support may be carried out by immersing a zeolite in a solution containing the above-mentioned metal, followed by heating the mixture while stirring, to remove water. Examples of the metal salt used include chloride and ammine complex. Although there is no particular limitation on the content of the metal, the manganese content is preferably about 0.2 to 2.5, more preferably 0.25 to 2.0, in terms of the MnO/A1 2 O 3 mole ratio. When the metal content is lower than the above-mentioned respective ranges, a satisfactory activity cannot be obtained. When the metal content is higher than the above-mentioned respective ranges, however, a greater effect will not be attained merely by excessive amounts of added metal. The sample containing the above-mentioned metal may be used as a catalyst after a pretreatment such as drying, calcination or the like. There is no particular limitation on the configuration, structure, etc., of the catalyst for purifying an exhaust gas according to the present invention, and the catalyst for purifying an exhaust gas may be in the form of, for example, a pellet or a honeycomb. The introduction of the metal element may be carried out after molding. The catalyst for purifying an exhaust gas according to the present invention may be used after being mixed with an alumina sol, a silica sol or a binder such as a clay mineral and then molded. Alternatively, the catalyst may be used after water is added to the catalyst to prepare a slurry, which is then coated on a honeycomb-structured base material made of alumina, magnesia, cordierite or the like. The exhaust gas to be purified by the catalyst of the present invention is an oxygen rich exhaust gas containing nitrogen oxides, carbon monoxide and hydrocarbons. The term "oxygen rich exhaust gas" used herein is intended to mean an exhaust gas containing oxygen in an amount exceeding the amount of oxygen necessary for completely oxidizing reducing components such as carbon monoxide and hydrocarbons, etc., contained in the exhaust gas. The catalyst of the present invention can efficiently purify an exhaust gas when hydrocarbons having one carbon atom are contained in the exhaust gas as a main component. In general, most of the hydrocarbons contained in an exhaust gas discharged from an engine wherein a liquid fuel is used, such as automobiles, have 2 or more carbon atoms, but the hydrocarbons contained in an exhaust gas discharged from an engine wherein a gas fuel is used, such as gas engines, are mainly a hydrocarbon having one carbon atom. In general, the reactivity of the hydrocarbon increases with an increase of the number of carbon atoms, and when the number of carbon atoms is 1, the reactivity is particularly low. The expression "exhaust gas wherein the hydrocarbons contained in the exhaust gas are mainly a hydrocarbon having 1 carbon atom" is intended to mean an exhaust gas such that 80% or more of the hydrocarbons contained in the exhaust gas are hydrocarbons having 1 carbon atom. Examples of such an exhaust gas include exhaust gases discharged, for example, from gas engines in a lean combustion system wherein a city gas is used as the fuel. There is no particular limitation on the hydrocarbon added to the exhaust gas, and the catalyst of the present invention can efficiently purify an exhaust gas when the hydrocarbon is methane or a gaseous hydrocarbon mixture composed mainly of methane. The "gaseous hydrocarbon mixture, composed mainly of methane" means a mixture wherein 80% or more of the hydrocarbons contained in the mixture are methane. Examples of such a gaseous mixture include various town gases. There is no particular limitation on the concentration of the hydrocarbon added to the exhaust gas, and the hydrocarbon may be added such that the content thereof is about 50 ppm to 1%, based on the entire exhaust gas. Although the amount of addition of the hydrocarbon may be further increased, the profitability and the percentage of hydrocarbon conversion are unfavorably reduced. EXAMPLES The present invention will now be described in more detail with reference to the following Examples, but is in no way limited to these Examples. Example 1: Preparation of Catalyst 1 A 200 g amount of an NH 4 type ZSM-5 having a silica/alumina ratio of 40 was charged to 1800 ml of a 0.25 M aqueous manganese acetate solution, and the mixture was stirred at 80°C for 20 h to effect the ion-exchange. After the mixture was subjected to solid-liquid separation, the zeolite cake was charged again to an aqueous solution having the same composition as that of the above-mentioned aqueous solution, to again conduct an ion exchange procedure. The mixture was then subjected to solid-liquid separation, and the solid was washed with 20 liters of pure water and dried at 110°C for 10 h, to give a catalyst 1. As a result of an elementary analysis of the catalyst 1, it was found that the manganese content was 1.02 times based on the alumina content. Example 2: Preparation of Catalyst 2 A catalyst 2 was prepared in the same manner as in Example 1, except that an aqueous manganese nitrate solution was used instead of the aqueous manganese acetate solution. As a result of an elementary analysis of the catalyst 2, it was found that the manganese content was 0.54 time based upon the alumina content. Example 3: Preparation of Catalyst 3 A 200 g amount of an NH 4 type ZSM-5 having a silica/alumina ratio of 40 was charged to 1800 ml of a 1.09 M aqueous barium chloride solution, and the mixture was stirred at 80°C for 16 h. After the mixture was subjected to solid-liquid separation, the solid was thoroughly washed with water, charged to 700 ml of a 0.23 M aqueous manganese acetate solution, and stirred at 80°C for 16 h. The slurry was then subjected to solid-liquid separation, the zeolite cake was charged to a freshly prepared aqueous solution having the above-mentioned composition, and the above-mentioned procedure was repeated. The resultant slurry was subjected to solid-liquid separation, and the solid was thoroughly washed with water and dried at 110°C for 10 h, to give a catalyst 3. As a result of elementary analysis of the catalyst 3, it was found that the barium content and the manganese content were respectively 0.66 time and 0.87 time, based upon the alumina content. Comparative Example 1: Preparation of Comparative Catalyst 1 A 1 kg amount of an NH 4 type ZSM-5 having a silica/alumina mole ratio of 40 was added to a 0.1 M copper acetate solution so that the number of copper atoms was one time the number of A1 atoms in the zeolite. Thereafter, a 2.5% aqueous ammonia was added thereto to adjust the pH value to 10.5, and the mixture was stirred at room temperature for 20 h, to thereby conduct an ion exchange treatment. This procedure was repeated twice, and the resultant solid was washed and dried at 110°C for 12 h to give a comparative catalyst 1. As a result of a chemical analysis, the comparative catalyst 1 was found to have a CuO/A1 2 O 3 mole ratio of 1.05. Comparative Example 2: Preparation of Comparative Catalyst 2 A 20 g amount of an NH 4 type ZSM-5 having a silica/alumina ratio of 40 was charged to 180 ml of a 0.23 M aqueous nickel acetate tetrahydrate solution, and the mixture was stirred at 80°C for 16 h. After the slurry was subjected to solid-liquid separation, the zeolite cake was charged to a freshly prepared aqueous solution having the above-mentioned composition, and the above-mentioned procedure was repeated. The slurry was subjected to solid-liquid separation, and the solid was thoroughly washed with water and dried at 110°C for 10 h, to give a comparative catalyst 2. The comparative catalyst 2 was subjected to chemical analysis and found to have an NiO/A1 2 O 3 mole ratio of 1.40. Example 4 (Evaluation Test 1 of Catalyst) Catalysts 1 to 3 and comparative catalysts 1 to 2 were press-molded and then crushed to regulate the size of granules thereof to 12 to 20 mesh, and an atmospheric fixed bed type reaction tube was packed with 1.2 g of each of the granular catalysts. The fixed bed was pretreated at 500°C for one hour, while passing air through the reaction tube, and thereafter, a gas (hereinafter referred to as "reaction gas") having the composition shown in Table 1 was passed through the reaction tube at a flow rate of 500 ml/min, to thereby determine the catalytic activities at 400°C and 500°C, respectively. The NO x and methane conversions at the respective temperatures after the state had become steady are given in Table 2. In all of the shown catalysts, the conversions of CO, and of the hydrocarbons except for methane, were substantially 100%. The NO x conversion can be determined by the following equation; the methane conversion was similarly determined. wherein NO x concentration at inlet of reaction tube; and NO x concentration at outlet of reaction tube. Composition of Reaction Gas NO 600 ppm CO 500 ppm CH 4 2920 ppm C 2 H 6 160 ppm C 3 H 8 121 ppm n-C 4 H 10 33 ppm i-C 4 H 10 14 ppm CO 2 7% O 2 5% H 2 O 15% N 2 balance Results of Evaluation Test 1 of Catalyst 400°C 500°C NO x CH 4 NO x CH 4 Catalyst 1 Mn 28 3 35 45 Catalyst 2 Mn 30 5 33 41 Catalyst 3 Mn + Ba 24 3 27 30 Comparative Catalyst 1 Cu 16 13 12 75 Comparative Catalyst 2 Ni 9 0 13 5 Example 5 (Evaluation Test 2 of Catalyst) The catalytic activity was measured in the same manner as in Example 4, except that a hydrocarbon mixed gas composed mainly of methane as shown in Table 4 was added in an amount of 5000 ppm to the reaction gas shown in Table 3. The NO x and methane conversions at respective temperatures after the state had become steady are given in Table 5. In all of the shown catalysts, the conversions of CO, and of the hydrocarbons except for methane, were substantially 100%. Composition of Reaction Gas NO 600 ppm CO 500 ppm CH 4 500 ppm CO 2 7% O 2 5% H 2 O 15% N 2 balance Composition of Mixed Gas CH 4 88.0% C 2 H 6 5.8% C 3 H 8 4.4% n-C 4 H 10 1.2% i-C 4 H 10 0.5% Results of Evaluation Test 2 of Catalyst 400°C 500°C NO x CH 4 NO x CH 4 Catalyst 1 Mn 42 2 57 35 Catalyst 2 Mn 38 2 46 35 Catalyst 3 Mn + Ba 30 3 38 25 Comparative Catalyst 1 Cu 18 10 20 80 Comparative Catalyst 2 Ni 11 0 20 0 Comparative Example 3 (Evaluation Test 3 of Catalyst) The catalytic activity was measured in the same manner as in Example 4, except that a reaction gas specified in Table 3 was used as such as the reaction gas. The NO x and methane conversions at respective temperatures after the state had become steady are given in Table 6. In all of the shown catalysts, the conversion of CO was substantially 100%. Results of Evaluation Test 3 of Catalyst 400°C 500°C NO x CH 4 NO x CH 4 Catalyst 1 Mn 16 5 13 25 Catalyst 2 Mn 18 6 16 32 Catalyst 3 Mn + Ba 17 8 15 30 Comparative Catalyst 1 Cu 15 15 8 100 Comparative Catalyst 2 Ni 7 0 11 10 As apparent from the above-mentioned catalyst evaluation results, the zeolite catalysts of the present invention can efficiently remove nitrogen oxides, carbon monoxide and hydrocarbons from an oxygen-rich exhaust gas containing nitrogen oxides, carbon monoxide and hydrocarbons. Further, when the major component of the hydrocarbons contained in the exhaust gas is a hydrocarbon having one carbon atom, the zeolite catalysts of the present invention can remove nitrogen oxides, carbon monoxide and hydrocarbons at a conversion rate higher than that attained by the comparative catalysts. Further, it is apparent that the nitrogen oxides can be removed at a very high conversion rate when the catalyst of the present invention is used, and at the same time, a hydrocarbon is further added to the exhaust gas, which renders the present invention very useful for environmental protection.

Claims: A method for purifying an exhaust gas to remove nitrogen oxides, carbon monoxide, and hydrocarbons from an exhaust gas containing the same as well as oxygen in an amount exceeding the amount of oxygen necessary for completely oxydizing reducing components such as carbon monoxide or hydrocarbons contained in the exhaust gas, comprising a step of bringing a catalyst into contact with the exhaust gas, said catalyst comprising a zeolite having manganese contained therein, with the provisio that neither potassium nor cesium is contained in the catalyst, and 80 % or more of said hydrocarbon being a hydrocarbon with only one carbon atom. A method as claimed in claim 1, wherein at least one hydrocarbon is further added to the exhaust gas. A method as claimed in claim 2, wherein the hydrocarbon added is methane or a mixture containing methane as a main component.

Inventors: KAWAI YOSHIAKI, KAWAI, YOSHIAKI
Applicants: TOSOH CORP, TOSOH CORPORATION
Application year: 1992
Main classification: B01D53
IPC codes: B01D53, B01J29