Method and catalyst for the purification of NOx-containing exhaust gases

Abstract: A method of purifying NOx-containing exhaust gases characterized by using ammonium acetate as a reducing agent to a zeolite-based catalyst. NOx in NOx-containing exhaust gases can be removed extremely effectively even at a low concentration thereof by using Fe-treated mordenite as a zeolite-based catalyst according to said method, and since it uses ammonium acetate as a reducing agent, an excellent effect free from ammonia slipping can be obtained. Particularly NOx in NOx-containing exhaust gases can be removed effectively at and around a discharge temperature of an exhaust gas from a gas engine in the presence of ammonium acetate as a reducing agent by constituting Fe-containing mordenite as a monolithic material. Hence, it makes it possible to obtain an excellent effect of simplifying the whole denitration system without needing temperature control of an exhaust gas and/or a catalyst bed. In addition, the NOx-purifying ability of the catalyst does not decrease over a long period of time.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique of purifying NOx-containing exhaust gases by means of a catalyst for purifying NOx-containing exhaust gases comprising a zeolite-based material in the presence of ammonium acetate as a reducing agent. 2. Background Art On incineration of industrial wastes and domestic refuse are formed NOx, CO, SOx, hydrochloride and offensive smells according to the origin, kind and composition of these wastes. Because of this, various measures have been taken to exhaust gases containing them and further research and development have been conducted; the same may be said of exhaust gases exhausted from cars, thermal-power generation systems and co-generation systems employing a gas engine. Particularly regarding the treatment of NOx (nitrogen oxides) of these harmful components in various exhaust gases, various methods have been known as so-called smoke-exhaustion denitration techniques including non-catalytic reduction, catalytic cracking, non-selective or selective catalytic reducton, adsorption, electron rays irradiation, molten salt absorption and reduction absorption. Of the above, catalytic reduction performing purification by means of a catalyst in the treatment usually converts NOx to N 2 finally to make it harmless and hence has been noted particularly. As catalysts to be used for the catalytic reduction have been known noble metals such as Pt, Rh and Pd, metal oxides such as TiO 2 , V 2 O 5 , Cr 2 O 3 and Fe 2 O 3 , rare earth metal oxides and sulfides, and zeolite-based catalysts. Zeolites are used effectively as a catalyst itself and also as a carrier for various catalysts. Zeolites, in general, are characterized by comprising aluminosilicate in composition, being crystalline and having a three-dimensional network, and containing cations with large ion-exchange properties. There are a lot of kinds thereof according to the ratio of an alumina (Al 2 O 3 ) component and a silica (SiO 2 ) component constituting it and the crystalline constitution thereof, and typical examples thereof include analcime, chabazite, mordenite, faujasite and clinoptilolite. In addition to natural zeolites, synthetic zeolites are produced according to hydrothermal synthesis and on the market. They are used broadly, for example, as fillers for paper, moisture-absorbing agents, ion exchangers, molecular sieves, catalysts for various reactions and carriers for catalysts, and denaturation, modification and other improvements thereof have been performed successively in accordance with various uses and reactions thereof. In the field of a catalyst for purifying NOx-containing exhaust gases, it has been proposed to deposit a metal such as copper and iron on zeolites for some improvements of the purification efficiency. EP 0 376 025 A1 and the corresponding Japanese laid-open patent publication 2-194819/1990 disclose a method for the reduction of nitrogen oxides from waste gas streams in a concentration of 800 ppm (vol.) NO x using ammonia as a reducing agent in combination with a zeolite catalyst. The catalyst is prepared by charging it with copper and/or iron and optionally cerium or molybdenum by ion exchange, impregnation or precipitating as well as by mixing metal oxide salts with the zeolite and subsequent calcination in an oxidizing or reducing atmosphere or in an evolving ammoniacal atmosphere. Therefore, metals such as Fe are only contained into ready-made zeolites by the aftertreatment and not contained in zeolites in the synthesis thereof. Besides, the official gazette of Japanese Laid-Open Patent Publication No. 2-4453/1990 proposes a method of producing a denitrating catalyst (namely, zeolite with a transition metal deposited thereon) for subjecting nitrogen oxides in an exhaust gas to catalytic reduction in the presence of ammonia. Here zeolite catalysts are produced by using Al, Si, compounds of alkali metals or alkali earth metals and salts of transition metals as materials, adjusting molar ratios of the material components to predetermined ratios and subjecting a gel solution with an effective alkali concentration adjusted to a hydrothermal treatment. Transition metals, as raw materials, are specifically Cu, Fe, Ni, Co and V and zeolites to be obtained here are mordenite and a pentacyl-type zeolite. In the above-mentioned catalytic reduction denitration purifying NOx in a NOx-containing exhaust gas by means of a catalyst, a reducing agent is indispensable. As reducing agents, ammonia, hydrogen, methane and other hydrocarbons, and carbon monoxide can be used; of them, since ammonia is excellent in selective reactivity to NO, it is now adopted to a practical use as practical denitration. The above official gazette of Japanese Laid-Open Patent Publication No. 2-194819/1990 and the official gazette of Japanese Laid-Open Patent Publication No. 2-4453/1990 make it essential to use ammonia as a reducing agent. In this point, the official gazette of Japanese Laid-Open Patent Publication No. 2-203923/1990 points out problems such as an ammonia leak, in case of using ammonia as a reducing agent similarly to the case of the above art and proposes the use of ammonium salts or amine compounds instead of it. Ammonium salts in the proposal are ammonium carbonate, ammonium bicarbonate, ammonium formate and ammonium acetate, and amino compound is urea. On the other hand, as catalysts in the proposal, known ones such as a carrier-depositing type, a non-carrier-depositing type and a Raney type can be used, and anatase-type titania with a metallic oxide such as V 2 O 5 deposited thereon exhibits particularly excellent denitration performance. However, there is no description regarding a zeolite-based catalyst and only this V 2 O 5 catalyst is described specifically in the proposal. Besides, the official gazette of Japanese Laid-Open Patent Publication No. 2-203923/1990 discloses, in the examples thereof, reaction ratios in case of introducing aqueous solutions of the above-exemplified reducing agents into a reaction tube, using "anatase-type titania with V 2 O 5 deposited thereon (deposition ratio of vanadium: 4 weight %)" as a catalyst at a react ion temperature within the range of 200 to 450 °C to an exhaust gas containing NO of 100 ppm prepared for an experiment. According to the above, in case of using ammonium acetate as a reducing agent, almost the same effect as in case of using ammonia described as Comparative Example there is obtained. However, these reaction ratios do not exceed the equivalent reaction of NO and a nitrogen atom in an ammonium salt. That is, in case of using ammonia as a reducing agent in catalytic reduction denitration and, as described above, also in case of using ammonium salts such as ammonium carbonate, ammonium acetate, ammonium bicarbonate and ammonium formate, the reaction between an N atom in the reducing agent and NO in the exhaust gas is below the equivalent, and hence NOx (NO) beyond the amount of the reducing agent (in terms of N) added cannot be removed. On the other hand, in case of treating an exhaust gas by means of a solid catalyst, the catalyst is employed 1 ○ in a form of a fluidized bed or a fixed bed as a particle, 2 ○ in a form of a fixed bed as a pellet, or 3 ○ in a form of a monolith deposited on a monolithic carrier; generally, they are selected according to the origin and kind of NOx-containing exhaust gases and other various requisites properly. However, there is no instance of showing that the form of a monolith in the above 3 ○ improves performance as compared with the form of the above 1 ○ or 2 ○. In the documents JP-A-06 319 954 and US-A-4 978 514 catalysts are disclosed that are made of general ceramic or general zeolite, regardless of the special composition of the catalyst according to the present invention as described in the following. SUMMARY OF THE INVENTION It is the object of the present invention to provide a method for the purification of exhaust gases containing NO x or NO x and CO even at a low concentration, capable of improving ratios of purifying NOx in exhaust gases sharply and also purifying CO effectively by using zeolite-based materials as a catalyst and ammonium acetate as a reducing agent in catalytic reduction denitration of exhaust gases. The present invention can also remove NOx beyond the added amount of the reducing agent by the use of the zeolite-based catalysts deposited a metal of a platinum group such as Rh. The catalyst for purifying NOx-containing exhaust gases according to the present invention comprises synthetic mordenite with Fe introduced into its structure by using an Fe-containing material on synthesis thereof, having particularly excellent catalytic characteristics even to an exhaust gas containing an extremely small amount of NOx and having an excellent NOx-purifying ability even under the condition of an extremely high space velocity, and a method of purifying NOx-containing exhaust gases by means of said catalyst in the presence of ammonium acetate as a reducing agent. The catalysis for purifying NOx-containing exhaust gases, which uses ammonium acetate as a reducing agent and exhibits a particularly excellent NOx-purifying ability in case of using Fe-containing mordenite synthesized according to the present invention as a denitrating catalyst by using it as a form of a monolithic material as compared with the case of using it as a particle or a pellet, and a method of purifying NOx-containing exhaust gases by means of the monolithic catalyst in the presence of ammonium acetate as a reducing agent. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows the relationship between the Fe content in production materials and the Fe content in the formed mordenite. Fig. 2 shows the relationship between the Fe content in the formed mordenite and NOx removal ratios. Fig. 3 is a sectional view showing an example of a monolithic catalyst to be applied in the present invention. Fig. 4 shows the change of treatment temperatures and NOx removal ratios in case of using a monolith-deposited catalyst and a pulverized particle catalyst. Fig. 5 shows the change of NOx removal ratios with tine elapsed of a treatment by means of a monolith-deposited catalyst. DETAILED DESCRIPTION First of all, the present invention is a method of purifying NOx-containing exhaust gases characterized by using a zeolite-based catalyst as a catalyst and ammonium acetate as its reducing agent in catalytic reduction denitration. Here exhaust gases may contain substantially NOx singly or may contain both NOx and CO as components to be purified according to the present invention. As the above catalyst to be used in the present invention is used synthetic zeolite. This synthetic zeolite has been determined as mordenite by X-ray diffraction but has particular characteristics due to its synthetic origin, which are responsable for its excellent purification properties. The components for the synthesis of the catalyst comprise sodium aluminate, sodium hydroxide, amorphous silica and an Fe-salt as main components. The synthesis takes place at 100 °C to 200 °C, preferably at 150 °C to 190 °C under spontaneous pressure in an autoclave or additional pressure under stiring at 300 rpm for approximately 72 hours. In addition, the effect of purifying NOx and CO can be more increased by depositing a metal of a platinum group on these catalysts according to an ordinary procedure. Moreover, according to the present invention, not only NOx can be removed but also CO in an exhaust gas is rapidly oxidized and removed as CO 2 effectively, and the effect of removing CO can be accomplished almost perfectly when a metal of a platinum group is deposited on the zeolite-based catalyst. Particularly Pt of a platinum group is prone to convert NO to NO 2 , and NO is consumed in an oxidization react ion and NO to be reduced to N 2 decreases; hence a preferable depositing metal is Rh or Pd. The present invention is performed by introducing an exhaust gas into a catalyst layer provided in a reaction tube. As a method of adding ammonium acetate as a reducing agent, the following various techniques may be employed: (1) introducing it into a stream of an exhaust gas at a catalyst layer or upstream by spraying or other techniques, (2) impregnating a zeolite-based catalyst with it in advance to deposit, and (3) employing both (1) and (2) in combination; it is advantageous to employ the technique of (1) with a view to performing the operation stably over a long period of time. In the above denitration described in the official gazette of Japanese Laid-Open Patent Publication No. 2-203923/1990 using an ammonium salt as a reducing agent, it is impossible to use the reducing agent in an amount exceeding the amount corresponding to the amount of N contained therein, namely, beyond the amount of the equivalent react ion with NO; according to the present invention, however, ammonium acetate as a reducing agent can remove NOx beyond the amount thereof added (in terms of N). However, the reducing agent in the present invention can be utilized beyond the amount of the equivalent reaction with NO in the presence of the zeolite-based catalyst, which exhibits extremely unusual characteristics and brings effective effects; but, the reason is unclear. It is guessed that not only ammonium content in ammonium acetate but also acetate content (CH3COO·) in it may have any effect upon the reaction. Thus, the present invent ion exhibits a removal effect beyond the equivalent amount to the amount of NOx in combination with ammonium acetate as a reducing agent with the zeolite-based catalyst, and can obtain an excellent effect free from slip ammonium shown in case of using ammonia as a reducing agent. The present invention can remove both NOx (NO) and CO in exhaust gases effectively by using a zeolite-based catalyst as a catalyst and ammonium acetate as its reducing agent in catalytic reduction denitration of exhaust gases and can further improve the removal effect by depositing a metal of a platinum group such as Rh on a zeolite-based catalyst. The present invention provides a method of purifying NOx-containing exhaust gases by means of synthetic mordenite with Fe contained in its constitution, which is obtained by using an Fe-containing material on synthesis thereof, and ammonium acetate as a reducing agent. The synthetic mordenite catalyst with Fe contained in its constitution according to the method of the present invention can be produced by allowing a material containing Fe such as Fe nitrate to exist as a material component itself in addition to an alumina material such as sodium aluminate, as silica material such as amorphous silica and an alkali metal compound such as caustic soda or an alkali earth metal compound as synthetic material components on the production of mordenite according to hydrothermal synthesis in an autoclave. Here as an alumina material may be used, in addition to sodium aluminate, alumina sol, alumina gel, aluminum sulfate, aluminum nitrate and aluminum hydroxide, and as a silica material nay be used amorphous silica, water glass, silica sol, silica gel, diatomaceous earth and clay minerals. In the present invention, an Fe-containing material is allowed to exist as a material component itself in addition to these materials, and as an Fe-containing material may be used Fe chloride, Fe bromide, Fe oxides, Fe sulfate and organic Fe compounds in addition to the above Fe nitrate. Regarding the ratio of an amount of each material, an amount corresponding to the composition capable of obtaining a reaction product with a mordenite constitution may be used. Fig. 1 (a) and (b) show the relationship between the change of the amount of an Fe material in production materials on the above synthesis and the amount of Fe contained in the mordenite product obtained according to the change; Fig. 1 (a) shows the "Fe/(Al + Fe)" molar ratio in the formed mordenite to the "Fe/(Al + Fe)" molar ratio in materials, and Fig. 1 (b) shows the Fe content (weight %) in the formed mordenite to the "Fe/(Al + Fe)" molar ratio in materials. As shown in Fig. 1 (a), Fe added into the materials is contained in the formed mordenite structure as it is till the "Fe/(Al + Fe)" molar ratio is less than 0.4 and hereinafter as the amount of Fe increases gradually, the ratio of Fe taken into the formed mordenite structure decreases ; however, as shown in Fig. 1 (b), Fe may be contained by around 5 weight % in the formed mordenite structure. In addition, though Na/Al (ratio of numbers of atoms) is 1 in an Na-type mordenite, Na is contained in the above mordenite structure with Fe present therein in an amount corresponding to Na/(Al + Fe) = 1, showing no change in the content. Fig. 2 shows the relationship between the Fe content in the synthetic mordenite structure of the present invention and NOx removal ratios by experiments [Si/Al molar ratio: 5 to 10; treatment temperature (= reaction temperature): 300 °C; conditions of a gas to be treated and space velocity are the same as in Example to be described later]; as shown in the drawing, a NOx removal ratio beyond 50 % is shown at an Fe content of 0.3 to 4.7 weight %, and a NOx removal ratio beyond 60 % can be obtained at an Fe content of 0.4 to 4.2 weight %. Here, a NOx removal ratio is calculated according to the following equation (1), in which the concentration of NOx before the treatment is shown as X and the concentration of NOx after the treatment as Y; the same is true of a NOx removal ratio in Examples. NOx removal ratio (%) = X - Y X x 100 Mordenite is one kind of aluminosilicates and has a rhombic form with a channel system therein: in the present invention, one with an Si/Al molar ratio in composition within the range of 5 to 50 (SiO 2 /Al 2 O 3 molar ratio: 10 to 100) may be employed effectively as a catalyst for NOx-containing exhaust gases. Hence the amounts of a silica material and an alumina material may be selected according to the final composition, and the above-described Fe material to be contained is added to use in an amount necessary for obtaining the characteristics shown in Fig. 2 according to the trend and facts shown in Fig.1(a) and (b). The synthesis in the present invention is performed according to hydrothermal reaction; it is performed under the heating condition of about 100 to 200 °C, preferably about 150 to 190 °C and under spontaneous pressure in an autoclave or under pressure beyond it. The reaction may be performed in a state left to stand; with a view to allowing Fe to exist surely and firmly in the structure of the formed mordenite and from the viewpoints of a yield and a reaction rate thereof, however, it is preferable to perform the reaction under the condition of stirring, particularly under mild stirring. In the operation of mixing materials, Fe can be surely present in the structure of the formed mordenite by adding an aqueous solution of an Fe-containing material into an aqueous solution containing an alumina material, a silica material and an alkali metal compound or an alkali earth metal compound slowly dropwise under stirring to obtain a uniform mixed solution. According to the above process of production, the obtained synthetic mordenite with Fe contained in its structure indicates excellent characteristics; the catalyst has excellent NOx-purifying ability also in case that the concentration of NOx in an exhaust gas is extremely low, and besides, extremely excellent catalytic characteristics of a NOx removal ratio of 60-78 % (and that at a relatively low treatment temperature of 300 to 325 °C) are exhibited under instantaneous, severe treatment conditions such as a space velocity of 220000 hr -1 . As a method of adding ammonium acetate as a reducing agent in case of performing the present invention by passing an exhaust gas through a catalyst layer provided in a reaction tube, the following various techniques may be employed: (1) introducing it into a stream of an exhaust gas at a catalyst layer or its upstream according to spraying or other techniques, (2) impregnating a zeolite-based catalyst with it in advance to deposit, and (3) employing both (1) and (2) in combination; it is advantageous to employ the technique of (1) with a view to performing the operation stably over a long period of time. In addition, the catalyst comprising synthetic mordenite with Fe contained in its structure on synthesis thereof according to the present invention has an excellent NOx-purifying ability even in case of an extremely low concentration of NOx in an exhaust gas, and hence can reduce and purify an extremely small amount of NOx in a dilute combustion exhaust gas from a gas engine and the like effectively; and besides, since it uses ammonium acetate as a reducing agent, an excellent effect free from slip ammonia shown in case of using ammonia as a reducing agent can be obtained. Fourthly, the present invention provides a catalyst for purifying NOx-containing exhaust gases comprising Fe-containing mordenite with ammonium acetate as a reducing agent, said Fe-containing mordenite being constituted as a monolithic material, and moreover provides a method of purifying NOx-containing exhaust gases by means of a monolithic catalyst composed of Fe-containing mordenite as a monolithic material and ammonium acetate as a reducing agent. Here, the above form of "Fe-containing mordenite constituted as a monolithic material" can be obtained by (1) forming Fe-containing mordenite into a monolithic shape by means of a binder such as silicate (water glass and the like) and phosphate, or (2) depositing Fe-containing mordenite on a monolithic carrier formed in advance. As a form of the monolithic material may be employed one with a honeycomb section and those with forms conventionally known as monolithic carriers; Fig. 1 (a) and (b) show enlarged sectional views of some examples to be used in the present invention. In case of (2) depositing Fe-containing mordenite on a monolithic carrier formed in advance, materials of the monolithic carrier are not particularly restricted; preferably ceramics or metals are used, and as ceramics may be used cordierite preferably and as metals may be used an iron-aluminum-chrome-based alloy and stainless steel preferably. As a method of depositing, a monolith-deposited catalyst can be obtained by mixing a catalyst powder with water and a binder such as alumina sol, and stirring the mixture to obtain an aqueous suspension, impregnating a monolithic carrier with the aqueous suspension (namely, wash-coating), removing excess adhered slurries by means of an air gun and the like, drying the resultant product, and calcinating it. Excess adhered materials are removed by means of an air gun and the like after the calcination on demand. The purification of a NOx-containing exhaust gas by means of a monolithic catalyst in the present invention is performed by providing the monolithic catalyst so that the monolithic opening in a reaction tube faces the direction of the flow of the exhaust gas, and passing the NOx-containing exhaust gas through the monolithic catalyst. The addition of ammonium acetate as a reducing agent is performed by introducing it into a stream of an exhaust gas just before the monolithic catalyst or at its upstream according to spraying or other techniques. In this case, NOx in the exhaust gas can be purified effectively from the start, namely, from the beginning of the operation of purification, by impregnating the monolithic catalyst with ammonium acetate in advance to deposit. As described above, the monolithic catalyst according to the present invent ion has NOx-purifying ability effective within a broad range of temperature and can be used effectively over a long period of time. Since it can remove NOx effectively at and around a discharge temperature of an exhaust gas from a gas engine, it makes it possible to obtain an excellent effect of simplifying the whole denitration system without needing temperature control of an exhaust gas and/or a catalyst bed. In addition, the monolithic catalyst according to the present invention has an excel lent NOx-purifying ability even in case of an extremely low concentration of NOx in an exhaust gas, and hence can reduce and purify an extremely small amount of NOx in an exhaust gas from a gas engine and the like effectively. Besides, since it uses ammonium acetate as a reducing agent, an excellent effect free from ammonia slipping shown in case of using ammonia or urea as a reducing agent can be obtained. Hereunder, Examples of the present invention will be described, but the present invent ion is limited to said Examples by no means. EXAMPLE I The present will show, first of all, an example of producing a synthetic mordenite catalyst with Fe contained in its structure in the process of production thereof, and then describe an example of an experiment of purifying a NOx-containing exhaust gas performed by using this catalyst. 〈Example of Production〉 First of all, as materials for product ion were prepared commercially available sodium aluminate, sodidum hydroxide and amorphous silica (Nipsil VN3; manufactured by Nippon Silica Kogyo; trade name) and as an Fe material was prepared Fe nitrate nonahydrate [Fe(NO 3 ) 3 9H 2 O]. Sodium aluminate and sodium hydroxide were put into distilled water in a beaker (volume: 100 0 ml) to stir and dissolve, and amorphous silica was introduced therein to make a liquid A; on the other hand, Fe nitrate nonahydrate was dissolved into distilled water in a beaker (volume: 500 ml) to make a liquid B. The quantity ratios of materials and distilled water in the liquid A and the liquid B are as shown in Table 3. Next, the liquid B was added into the total amount of the liquid A dropwise slowly and stirred to make a uniform mixed solution, which was transferred into a stainless steel autoclave equipped with a rotary stirrer and a Teflon® vessel in the inside (inner volume: 1000 ml) and sealed, and then a hydrothermal reaction was performed. The reaction was performed under stirring (300 rpm) maintaining a temperature of 180 °C, and after the treatment condition was maintained three whole days and nights (72 hours), heating was stopped to leave it to stand and the reaction product was cooled to normal temperature according to natural cooling. Subsequently, the reaction product was filtered, washed with water till the pH of a slurry became less than 10, and dried at 80 °C overnight to obtain a powdery reaction product. The reaction product was subjected to X-ray diffraction (XRD) and each sample showed a clear mordenite structure (Na type). As a result of fluorescent X-ray diffraction, it was revealed that each of samples with Fe nitrate nonahydrate added in synthetic materials thereof contained a predetermined amount of Fe in its structure. The above conditions and results of each sample are shown in Table 1. In Table 1 a case of production without adding the liquid B is shown as Comparative Example. Subsequently, to each of the Na-type Fe-containing mordenites (reaction products of Examples 1-8 in Table 1) were added 0.5 mol/l(81g/1000 ml) of an aqueous ammonium nitrate solution (about 5 times equivalent of Na = 500 ml/Na-type Fe mordenite 20 g), and the mixed solution was stirred at 60 °C for one hour, filtered and washed With water; this treatment was repeated again and then reaction product was dried at 110 °C for about 5 hours. Further it was calcinated at 500 °C for 2 hours, and an H-type Fe-containing mordenite of each sample was obtained and used as a specimen catalyst in the following experiment of purifying NOx-containing exhaust gases. An H-type mordenite was obtained similarly in Comparative Example. Each sample of these Na-type Fe-containing mordenites converted to H-type was subjected to fluorescent X-ray diffraction similarly as above, and no change of Fe content was observed. Cleaning Test Using 0.5 ml of each of the thus obtained specimen catalysts (catalysts obtained by converting the react ion products 1-8 and Comparative Example in Table 1 to H-type), an experiment of purifying a NOx-containing exhaust gas was performed. As an exhaust gas to be treated was used a gas containing NO of 48 ppm, CO of 910 ppm, CO 2 of 6.8 %, O 2 of 9.1 %, steam of 9.1 % and N 2 as balance, and the experiment was conducted at a treatment temperature of 250 to 400 °C and at a space velocity of 220000 hr -1 . A fixed bed running-type react ion device was used as a reaction device in the experiment of purification; and ammonium acetate as a reducing agent was sprayed into the reaction tube just before a catalytic layer as an aqueous solution thereof and introduced into it so that the amount of the reducing agent should be 36 ppm (in terms of N) in the exhaust gas to be treated. The results are shown in Table 4. In Table 2, NOx removal ratios were measured at reaction temperatures of 250, 300, 325, 350, 375 and 400 °C. Comparative Example 1 used mordenite containing no Fe in its structure produced without adding any Fe material in the process of production thereof. Further in Table 2, Comparative Examples 2-3 performed an experiment of purification according to the method of the above Japanese Laid-Open Patent Publication No. 2-203923/1990 by using a catalyst produced according to Examples described there and ammonium acetate as a reducing agent. The catalyst was obtained by dipping 40 g of TiO 2 (anatase-type) in an aqueous ammonium metavanadate solution (3.57 g/300 ml of water) overnight, subsequently removing water at a temperature of 50 °C under vacuum by means of an evaporator and calcinating the resultant product at 400 °C for 3 hours. The vanadium deposition ratio of the thus obtained catalyst was 3.2 weight %. Comparative Example 2 was performed at a space velocity of 44000 hr -1 , while Comparative Example 3 at a space velocity of 220000 hr -1 . As shown in Table 2, it is apparent that Examples 1-8, namely, the mordenite catalysts With Fe contained in their structures on synthesis thereof, can remove and purify NOx in an exhaust gas extremely effectively in the presence of ammonium acetate as a reducing agent even at an extremely low concentration of 48 ppm of NO content in an exhaust gas, and that they show excellent characteristics at a relatively low temperature of 300 to 325 °C. For example, while the NOx removal ratios in Comparative Example 1 (H-type mordenite with no Fe contained therein) at reaction temperatures of 300 °C and 325 °C are only 7.4 % and 9.3 % respectively, those of Example 3 are 64.8 % and 59.3 % and further those of Example 5 are 77.4 % and 60.0 %, both showing high ratios. Since they are values obtained under instantaneous, severe conditions of a space velocity of 220000 hr -1 , it is apparent that they show an extremely excellent effect of removing NOx. Moreover, though Comparative Example 2 also uses ammonium acetate as a reducing agent, it exhibits a considerable denitration ratio under a relatively mild condition of a space velocity of 44000 hr -1 (though the highest denitration ratio is 45.5 % at a temperature of 300 °C); on the other hand, though Comparative Example 3 performed at a space velocity of 220000 hr -1 also uses ammonium acetate as a reducing agent, it only exhibits the highest denitration ratio of 12.7 % (250, 300 °C) to fail to obtain a desired denitration effect. EXAMPLE II The present will describe an example of producing an Fe-containing synthetic mordenite catalyst (an example of producing a synthetic mordenite catalyst with Fe contained in its structure in the process of production thereof), an example of preparing a monolith-deposited catalyst with said catalyst deposited on a monolithic carrier and a pulverized particle catalyst as Comparative Example, and an example of an experiment of purifying NOx-containing exhaust gases performed by menas of them. 〈Example of Production of an Fe-Containing Synthetic Mordenite Catalyst〉 First of all, as materials for production were prepared commercially available sodium aluminate, sodidum hydroxide and amorphous silica (Nipsil VN3; manufactured by Nippon Silica Kogyo; trade name) and as a Fe material was prepared Fe nitrate nonahydrate [Fe(NO 3 ) 3 9H 2 O]. Of them, 23.46 g of sodium aluminate and 9.07 g of sodium hydroxide were put into 440 g of distilled water in a beaker (volume: 1000 ml) to stir and dissolve, and 81.80 g of amorphous silica was introduced therein to make a liquid A; on the other hand, 12.12 g of Fe nitrate nonahydrate were dissolved into 100 g of distilled water in a beaker (volume: 500 ml) to make a liquid B. Next, the liquid B was added into the total amount of the liquid A dropwise slowly and stirred to make a uniform mixed solution, which was transferred into a stainless steel autoclave equipped with a rotary stirrer and a Teflon® vessel in the inside (inner volume: 1000 ml) and sealed, and then a hydrothermal reaction was performed. The synthesis was performed under stirring (300 rpm) maintaining a temperature of 180 °C, and after the treatment condition was maintained three whole days and nights (72 hours), heating was stopped to leave it to stand and the reaction product was cooled to normal temperature according to natural cooling. Subsequently, the reaction product was filtered, washed with water till the pH of a slurry became less than 10, and dried at 80 °C overnight to obtain a powdery react ion product. SiO 2 /Al 2 O 3 (molar ratio) of the reaction product was 13.3 and Fe content was 1.5 weight %. Subsequently, to the Na-type Fe-containing mordenite were added 0.5 mol/l (81 g/1000 ml) of an aqueous ammonium nitrate solution (about 5 times equivalent of Na = 500 ml/Na-type Fe mordenite 20 g), and the mixed solution was stirred at 60 °C for one hour, filtered and washed with water; this treatment was repeated again and then the reaction product was dried at 110 °C for about 5 hours. Further it was calcinated at 500 °C for 2 hours to obtain an H-type Fe-containing mordenite. 〈Example of Preparation of a Monolith-Deposited Catalyst〉 The Fe-containing mordenite powder obtained according to the above 〈Example of production〉 was deposited on a monolithic carrier as below. As a carrier was used a cordierite honeycomb carrier obtained by cutting a carrier with a sectional form shown in Fig. 1 (a) into a column, with a diameter of 20 mm⊘ (radius r = 10 mm = 1 cm), a length L of 23 mm and a cell number of 200/inch square. The volume of the honeycomb was 7.2 ml ( π r 2 L = 3.14 x 1 2 x 2.3 = 7.2 cm 3 ) and the total surface area thereof coming into contact with an exhaust gas was 1.59 x 10 -2 m 2 when it was set in a reactor to use. On the other hand, 20 g of the above catalyst powder were mixed with 30 g of distilled water and 20 g of alumina sol (Alumina Sol-200, manufactured by Nissan Chemical Industries, Ltd. ; trade name) and stirred to obtain an aqueous suspension. The above cordierite honeycomb carrier was impregnated with the aqueous suspension for 30 seconds to wash-coat, then excess adhered slurries were removed by means of an air gun, and the resultant product was maintained at 115 °C for one hour and dried. The treatment was repeated three times. Subsequently, the dried product was maintained at 115 °C for 6 hours and then calcinated at 500 °C for 2 hours, and excess adhered materials were removed by means of an air gun to obtain a monolith-deposited catalyst. The catalyst deposition amount of this honeycomb-deposited catalyst was 116.0 g/l. 〈Example of Preparation of a Particulate Catalyst = Comparative Example〉 The Fe-containing mordenite powder obtained according to the above 〈Example of Production〉 was introduced into a press, pressed at a pressure of 500 kg/cm 2 and pulverized. The pulverized product was made a pulverized particle with a particle diameter of 355-710 µm (average particle diameter: 533 µm) by means of a sieve and used as a specimen catalyst for Comparative Example. The pulverized particle had a volume density on charging of 0.576 g/ml, a real density of 0.96 g/ml with a porosity of 0.4, and hence a geometric surface area of 1.17 x 10 -2 m 2 /g. Namely, when 0.5183 g of the pulverized particle catalyst is set into a reaction tube as a catalyst bed to use, the total surface area coming into contact with an exhaust gas is 6.06 x 10 -3 m 2 . Cleaning Test Using the thus obtained monolith-deposited catalyst and 0.9 ml of the above specimen catalyst for Comparative Example, an experiment of purifying a NOx-containing exhaust gas was performed. As a reaction device was used a tubulous fixed bed running-type react ion device with an inner diameter of 20 mm and the above monolith-deposited catalyst was inserted into the reaction tube to fix; for the specimen catalyst for Comparative Example (pulverized particle) was used a tubulous fixed bed running-type reaction device with an inner diameter of 10 mm and the pulverized particle catalyst was held layer-like in the reaction tube by means of glass wool. As an exhaust gas to be treated was used a gas containing NO of 48 ppm, CO of 910 ppm, CO 2 of 6.8 %, O 2 of 9.1%. steam of 9.1 % and N 2 as balance, and the experiment was conducted at a treatment temperature of 250 to 450°C. The amount of the exhaust gas to be treated was determined so that a space velocity should be 66000 hr -1 in the monolith-deposited catalyst and 220000 hr -1 in Comparative Example. Ammonium acetate as a reducing agent in the purification experiment was sprayed into the react ion tube just before a catalyst bed as an aqueous solution thereof and introduced into it so that the amount of the reducing agent should be 36 ppm (in terms of N) in the exhaust gas to be treated, and NOx removal ratios were measured at reaction temperatures of 250, 300, 325, 350, 375, 400, 425 and 450 °C. The results are shown in Table 2. As shown in Table 2, Comparative Example shows a peak of a NOx removal ratio of over 70 % at a react ion temperature of 300 °C, but the ratio decreases at a lower temperature region and a higher temperature region rapidly. On the other hand, the NOx removal ratio in Example using a monolith-deposited catalyst reaches about 70 % around 325 °C and can be maintained almost the same at a reaction temperature of 450 °C. An exhaust gas from a gas engine is discharged generally at a temperature of the order of 300-500 °C; if the exhaust gas is discharged, for example, at 350 °C, it is necessary to cool and control the temperature of the exhaust gas and/or the catalyst bed to about 300 °C in order to allow the catalyst of Comparative Example to function effectively; it is apparent, however, that the monolith-deposited catalyst according to the present invention can purify NOx effectively without needing such cooling. The space velocity of Example II is different from that of Comparative Example; the "passing rates of an exhaust gas per contact area" of both catalysts calculated according to the above values are as below. Both values were obtained under almost the same conditions. (1) Passing rate of a gas in Example IV: Vh (m/h) = 66000 x 7.2 x 10 -6 /1.59 x 10 -2 = 29.9 (m/h) (2) Passing rate of a gas in Comparative Example: Vp (m/h) = 220000 x 0.9 x 10 -6 /6.06 x 10 -3 = 32.6 (m/h) EXAMPLE III Using a honeycomb-deposited catalyst produced according to the same procedure as in Example II and setting it in the same fixed bed running-type reaction device as in Example II, a durability experiment of the purification treatment of a NOx-containing exhaust gas was performed in the present Example III. As an exhaust gas to be treated was used a gas containing NO of 90 ppm, CO of 910 ppm, CO 2 of 6.8 %, O 2 of 9.1 %, steam of 9.1 % and N 2 as balance, and the expriment was conducted at a treatment temperature of 375 °C. Ammonium acetate as a reducing agent was sprayed into the reaction tube just before a catalyst layer as an aqueous solution thereof and introduced into it so that the amount of the reducing agent should be 68 ppm (in terms of N) in the exhaust gas to be treated. Fig. 5 shows the results of performing an experiment at a space velocity of the gas to be treated of 660000 hr -1 . As shown in Fig. 5, the honeycomb-deposited catalyst according to the present invention showed a NOx removal ratio of nearly 70 % from the beginning, showed no change in the NOx removal ratio even after 150 hours and maintained a high NOx-removing ability. The features disclosed in the foregoing description, in the claims and/or in the accompanying drawings may, both separately and in any combination thereof, be material for realising the invention in diverse forms thereof.

Claims: A method for reducing NO x contained in an exhaust gas comprising contacting a NO x -containing gas with a mordenite catalyst containing Fe in the presence of ammonium acetate as reducing agent, thereby reducing said NO x , characterized in that said mordenite is synthesized in a hydrothermal reaction. A method according to claim 1, wherein said mordenite is synthesized from components comprising sodium aluminate; alumina sol, alumina gel, aluminum sulfate, aluminum nitrate, aluminum hydroxide, water glass, silica sol, silica gel, diotomaceous earth, clay minerals, sodium hydroxide, amorphous silica and an Fe-salt, e. g. Fe-nitrate, Fe-chloride, Fe-oxide, Fe-bromide. A method according to one of the preceding claims, wherein said mordenite contains Fe in an amount of 0.3 to 4.7 weight-%. A method according to one of the preceding claims, wherein the said mordenite is synthesized at a temperature of 100°C to 200°C, preferably at 150°C to 190°C under the spontaneous pressure in an autoclave or under pressure beyond it. A method according to one of the preceding claims, wherein said mordenite is synthesized without stirring, preferably under mild stirring at e.g. 300 rpm for approximately 72 hours. A method according to one of the preceding claims, wherein said mordenite is cooled slowly to room temperature following the hydrothermal reaction. A method according to one of the preceding claims, wherein said mordenite is transformed to a monolithic body. A method according to one of the preceding claims, wherein said mordenite is deposited onto a monolithic carrier material. A method according to claim 8, wherein said carrier material consists of ceramics or metal. A method according to one of the preceding claims, wherein said mordenite contains platinum group metals in addition to Fe. A method according to one of the preceding claims, wherein ammonium acetate is sprayed into a stream of NO x -containing exhaust gas as an aqueous solution.

Inventors: SATOKAWA SHIGEO, TAKAHASHI IRURU, UCHIDA HIROSHI, YAMASEKI KEN-ICHI, SATOKAWA, SHIGEO, TAKAHASHI, IRURU, UCHIDA, HIROSHI, YAMASEKI, KEN-ICHI
Applicants: TOKYO GAS CO LTD, TOKYO GAS CO., LTD.
Application year: 1995
Main classification: B01D53
IPC codes: B01D53