PROCESS FOR PREPARING A CATALYST FOR REMOVING NITROGEN OXIDES

Abstract: A process for preparing a catalyst for removing nitrogen oxides which comprises sintering orthotitanic acid in coexistence of at least one compound, selected from silicic acid in the form of fine particles, tungsten compounds and molybdenum compounds.

Description: This invention relates to a process for preparing a catalyst for removing nitrogen oxides and more particularly, to a process for preparing a catalyst which does not oxidize sulfur dioxide on removal of nitrogen oxides. It has been already known that a titanium oxide sintered product is used as a carrier or catalyst. Since the surface area, crystal form, mechanical strength and heat resistance, which give an important influence on the performance of the carrier or catalyst, vary depending on the manner of preparation, the absence or presence, type and amount of additive, various processes have been heretofore provided. For instance, it is known from DE-A-28 46 476, that by coating titanium oxide with a layer of molybdenum oxide and by kneading a powder of titanium oxide with a small amount of water an improved mechanical strength and a high surface-to-volume ratio is attainable. As this titanium oxide works as a binder and molybdenum and tungsten are added and kneaded while titanium is in its oxidized state (i.e. in a powdery state) that means kneading a mixture of solid and liquid. Further it is known from FR-A-25 53 679 to obtain a durable catalyst which is active in denitration and inactive in the oxidation of SO₂. This is acheived by using, as a catalytic component, a sulfur-containing oxide mixture which contains SiO₂ as well as TiO₂. In this specification, molybdenum and tungsten are added to solid TiO₂ which is in the form of a formed oxide mixture. This also means kneading a mixture of solid and liquid. In both cases molybdenum arid tungsten in the liquid state are mixed into a powder of titanium which is the maincarrier and starts with a stable crystalline state and does not change its condition even after the addition of molybdenum and tungsten. According to the present invention the compounds of the mixture are in the liquid state, what improves the uniformity of the mixture thus the performance of the catalyst, which has a higher rate of removal of nitrogen oxides. Further, it is known that metatitanic acid or orthotitanic acid, which is readily obtained by thermal hydrolysis of titanates, is converted by sintering into anatase-type titanium oxide. This type of titanium oxide is generally, though not essentially, considered to be a favorable crystal form for use as a carrier or catalyst and has been used as a starting material for a carrier on which an ordinary catalyst for removing nitrogen oxides is to be supported. In a method in which silica is added to the thus obtained titanium hydroxide or titanium oxide and sintered, however, it is difficult to obtain a mixture of a uniform composition. Especially, where silica is added to titanium hydroxide which is in the form of a gel, it is not possible to uniformly disperse silica to titanium hydroxide. Thus, there cannot be obtained a carrier or catalyst of a high performance. According to a method in which an additive, such as silica, is added to orthotitanic acid, a mixture of a uniform composition is also difficult to obtain. Since orthotitanic acid is generally in the form of a gel, it is difficult to uniformly disperse an additive to orthotitanic acid, so that there cannot be obtained a carrier or catalyst of a high performance. It is an object of the invention to provide a process for preparing a catalyst or carrier useful for removing nitrogen oxides which overcomes the above-mentioned drawbacks, which has a better performance than those obtained by prior art methods. Broadly, the present invention provides a process for preparing a catalyst for removing nitrogen oxides which comprises sintering orthotitanic acid in coexistence of at least one compound selected from the group consisting of finely divided tungsten compounds and molybdenum compounds. Thereto, the present invention provides a process for preparing a catalyst for removing nitrogen oxides which comprises that active components for removal of nitrogen oxides may be further supported on the said sintered product. FIG. 1 is an X-ray spectrum of a sintered titanium product obtained in Example 7; FIGS. 2 is an X-ray spectrum of a sintered titanium product obtained in Example 14; FIG. 3 is an X-ray spectrum of a titanium oxide pigment for comparison. According to one embodiment of the invention, there is provided a process for preparing a carrier for a catalyst which is adapted for removal of nitrogen oxides, the process comprising adding a finely divided tungsten compound and/or a molybdenum compound to titanium sulfate, subjecting the mixture to neutralization and hydrolysis to form orthotitanic acid, drying the resulting mixture, and sintering the dried mixture to obtain a sintered product and a process for preparing a catalyst for removal of nitrogen oxides comprising that catalytic components for removal of nitrogen oxides are deposited on the said sintered product. In the above process characterized by adding a finely divided tungsten compound and/or molybdenum compound to titanium sulfate subjecting the mixture to neutralization and hydrolysis, drying and then sintering, it is preferred that the orthotitanic acid obtained by neutralization and hydrolysis of titanium sulfate in the presence of a finely divided tungsten compound and/or molybdenum compound is converted into a sol, filtered and dried and then sintered. The process according to this embodiment is characterized by sintering a sol of orthotitanic acid which contains at least one compound selected from tungsten compounds and molybdenum compounds, and is characterized that catalytic components for nitrogen oxides may be supported or deposited on the resulting sintered product to obtain a catalyst for removal of nitrogen oxides. According to another embodiment of the invention, orthotitanic acid is sintered in the presence of a finely divided tungsten compound and/or a molybdenum compound, by which crystal growth of titanium oxide is suppressed at the time of sintering of orthotitanic acid so that crystals of titanium oxide are left ungrown as anatase. The resulting sintered product has thus a large surface area and improved mechanical strength and heat resistance. This sintered product may be as it is or by depositing a certain type of metal oxide on the product as a carrier. If a metal oxide is supported on the sintered product, there is obtained a catalyst for nitrogen oxides which is significantly improved, as have never been achieved in prior art, by the synergistic effect of the oxides in the carrier and the metal oxide used. The process according to this embodiment of the invention comprises sintering orthotitanic acid in the form of a sol which contains at least one compound selected from finely divided tungsten compounds and molybdenum compounds, or comprising that catalytic components for nitrogen oxides may be deposited or supported on the resulting sintered product. The tungsten compounds used in the present invention are tungsten oxide or precursors capable of conversion into tungsten oxide by sintering. Examples of the precursors include ammonium paratungstate. Likewise, the molybdenum compounds include molybdenum oxide or precursors capable of conversion into molybdenum oxide by sintering. Examples of the precursors include ammonium molybdenate. Taking into consideration uniform mixing with anatase-type titanium oxide, these compounds should preferably be in the form of a solution. The amounts of the tungsten and molybdenum compounds are, respectively, from 5 to 50 wt%, calculated as WO₃ and MoO₃, on the basis of titanium oxide. Amounts less than 5 wt% result in a poor effect of suppressing the crystal growth at the time of sintering of orthotitanic acid. Over 50 wt%, a further effect of the addition is not recognized, thus being unfavorable from the standpoint of economy. In the practice of the invention, it is preferred that orthotitanic acid is converted to a sol, to which at least one compound selected from tungsten compounds and molybdenum compounds is added, followed by mixing and sintering. In accordance with a first embodiment of the invention, a finely divided tungsten compound is added to an aqueous solution of titanium sulfate and the mixture is neutralized for hydrolysis to produce orthotitanic acid, followed by co-precipitating the orthotitanic acid along with the finely divided tungsten compound. The substances used for the neutralisation and hydrolysis may be any basic substances, of which aqueous ammonia is preferred because of the ease in washing with water after the reaction. It is known that the neutralization and hydrolysis of titanium sulfate results in formation of orthotitanic acid. This orthotitanic acid is in the form of a gel. Preferably, after the neutralization and hydrolysis of titanium sulfate in the presence of the finely divided tungsten compound, the resulting gel of orthotitanic acid is partially or wholly solated. This allows the finely divided tungsten compound to be more uniformly dispersed in the orthotitanic acid sol. The manner of solation is not critically limited. For instance, the reaction mixture obtained by the neutralization and hydrolysis is washed with water to remove a substantial amount of the sulfate radicals, to which hydrochloric acid or nitric acid is added to solate part or substantially all of the reaction mixture. In case where the sulfate radical are not removed by washing with water, there are added to the reaction mixture chlorides of alkaline earth metals such as barium chloride, strontium chloride, calcium chloride and the like or nitrates of alkaline earth metals such as barium nitrate, strontium nitrate, calcium nitrate and the like. While the sulfate radicals are fixed as water-insoluble alkaline earth metal salts, the reaction mixture is partially or wholly solated. The amount of these solating agents is appropriately determined depending on the degree of solation of the reaction mixture. It will be noted that because the orthotitanic sol is subsequently gelled at a pH of 1 to 2 or higher, the sol may be gelled, if necessary, after uniform and sufficient mixing with at least one compound selected from finely divided, tungsten compounds and molybdenum compounds. The thus obtained mixture of orthotitanic acid and at least one compound selected from finely divided, tungsten compounds and molybdenum compounds is washed with water, filtered, dried and then sintered at a temperature not higher than 800°C, preferably from 700 to 200°C, followed by milling to obtain a sintered powder product. Since orthotitanic acid obtained by neutralization and hydrolysis of titanium sulfate is used, a slight amount of sulfate radicals is contained. This is considered to be one of factors for the formation of anatase-type titanium oxide which is preferred as a carrier for nitrogen oxide-removing catalysts. When the powder product is shaped into a desired form such as a honeycomb for use as a carrier, the dry product obtained by drying the said mixture before sinterning may be shaped by any known techniques such as, for example, an extrusion molding, a rolling granulation and the like, and subsequently sintered. Alternatively, the sintered powder product may be mixed with a suitable amount of water, kneaded and shaped into a desired form, followed by sintering again. In this case, after shaping into the desired form, the shaped articles are sintered at a temperature not higher than 800°C, preferably from 700 to 200°C. Thus, a titanium oxide sintered product in the form of powders or moldings can be obtain according to the invention. According to a further embodiment of the invention, a sintered product, which is obtained by neutralizing and hydrolyzing purified titanium sulfate (TiOSO₄) with a basic substance such as ammonia, washing the resulting mixture with water and sintering orthotitanic acid (Ti(OH)₄), is mixed with a tungsten compound and/or a molybdenum compound, dried and then sintered. The resulting product has a larger surface area and good strength and heat resistance and is thus very suitable for use as a catalyst for removal of nitrogen oxides or a carrier for various catalysts. Accordingly, a method for preparing a catalyst for removal of nitrogen oxide according to the embodiments of the invention is characterized by a process mixing a sintered product obtained by neutralization and hydrolysis of titanium sulfate with a tungsten compound and/or a molybdenum compound, and drying and sintering the mixture. Catalytic components for removal of nitrogen oxides may be supported on the final sintered product to obtain a catalyst for nitrogen oxides. The basic substances used for the neutralization and hydrolysis of the titanium sulfate may be any compounds which are basic. In view of the ease in removal by washing with water after the reaction, aqueous ammonia is preferred. The precipitate of the resulting orthotitanic acid is washed with water, filtered, dried, and sintered at a temperature not higher than 800°C, preferably from 700 to 300°C, and milled to obtain a sintered product in the form of powder. In this case, orthotitanic acid which is obtained from titanium sulfate is used, so that sulfate radicals are contained in slight amounts, resulting in formation of anatase which is preferred as a carrier or catalyst. Similar to the foregoing embodiments, a suitable amount of water is added to a mixture of the anatase and a tungsten compound and/or a molybdenum compound, kneaded and shaped into a desired form by any known techniques mentioned before, and sintered. If the tungsten compound and/or molybdenum compound is used in the form of a solution, water may not necessarily be required. The sintering temperature is in the range not higher than 800°C, preferably from 700 to 300°C. Thus, a sintered product of titanium oxide, tungsten oxide and/or molybdenum oxide can be obtained in the form of a powder or a molding. In the method according to any of the embodiments of the invention, when a powder of the dry or sintered product is mixed with an orthotitanic acid sol or gel, shaped into a desired form and sintered, various physical properties such as mechanical strength, porosity, specific surface area, pore distribution and the like can be further improved. In addition, the rate of shrinkage at the time of sintering can be suppressed. For this purpose, the amount of orthotitanic acid sol or gel to be added is conveniently in the range of from 5 to 50% of the final article on calculation as titanium oxide. Of course, for molding, ordinary molding aids may be used including methyl cellulose (Abicell, commercial name of Asahi Kasei Co., Ltd.). The sintering atmosphere is not critical and air, combustion gases, inert gases are all usable in the practice of the invention. The sintered product obtained by the invention is made of ungrown anatase, which is confirmed through X-ray spectra of FIGS. 1 and 2 having low and broad peaks. With anatasetype titanium oxide for pigments, the crystals grow to a substantial extent, as will be seen from the X-ray spectrum of FIG. 3, with a high and sharp peak. The sintered product of titanium oxide according to the invention may be used as a catalyst for removing nitrogen oxides as it is or as a carrier. When used as a carrier, the sintered product is supported with oxides which are known to have catalytic activity against nitrogen oxides, by which there can be obtained a catalyst which shows a good selective catalytic reduction performance for nitrogen oxides, using ammonia as a reducing agent, owing to unexpected synergistic effects with the oxides of the sintered product. The catalytic component to be supported in the present invention may be an oxide of at least one element selected from vanadium, tungsten, molybdenum, copper, iron, chromium, manganese and cerium. The oxide may be deposited on the titanium oxide sintered product by any known methods of preparing a catalyst. For instance, a solution or dispersion of the oxide or a precursor thereof is impregnated in or coated on the sintered product of a desired form and, if necessary, sintered at a predetermined temperature. The sintered product in the form of a powder may be mixed with the solution or dispersion, kneaded, shaped into a desired form and, if necessary, sintered at a predetermined temperature to obtain a catalyst for removal of nitrogen oxides. In order to remove nitrogen oxides from a mixed gas containing the nitrogen oxides by the use of a catalyst obtained by the invention, ammonia is added to the mixed gas in an amount of 0.5 to 5 times by mole, preferably from 1 to 2 times by mole, the nitrogen oxides to be removed and is passed through a reaction bed charged with the catalyst. The reaction bed may be a moving bed, a fluidized bed or a fixed bed. The catalyst obtained according to the invention contains at least one compound selected from finely divided tungsten compounds and molybdenum compounds and has so a good heat resistance that the reaction temperature may be in a wide range of from 200 to 600°C, preferably from 300 to 500°C. The space velocity is in the range of from 1,000 to 100,000 hr⁻¹, preferably from 3,000 to 30,000 hr⁻¹. Although the catalyst may be used to treat any gases containing nitrogen oxides, it can be used conveniently to remove nitrogen oxides from an exhaust gas from a boiler which contains 100 to 1000 ppm of nitrogen oxides chiefly composed of nitrogen monoxide, 200 to 2000 ppm of sulfur oxides mainly composed of sulfur dioxide, 1 to 10% by volume of oxygen, 5 to 20% by volume of carbon dioxide and 5 to 20% by volume of steam. According to the method of the invention, at least one compound selected from tungsten compounds and molybdenum compounds is allowed to be present in solated orthotitanic acid. In the mixture, the at least one compound is uniformly mixed. When the mixture is sintered, titanium oxide is left as ungrown anatase crystals by the action of the at least one compound. The resulting sintered product has a large surface area and significantly improved mechanical strength and heat resistance. When the sintered product is used as a catalyst for removal of nitrogen oxides with or without deposition of metal oxides having catalytic activity not only the catalytic activity of removing nitrogen oxides can be held over a long time under severe use conditions, but also the rate of oxidation of sulfur dioxide into sulfur trioxide is very low. This is considered to result from the synergistic effect of the anatase-type titanium oxide whose crystal development is suppressed and the at least one compound. Thus, the titanium oxide obtained by the invention is very useful for industrially removing nitrogen oxides. The present invention is more particularly described by way of examples, which should not be construed as limiting the invention. Example 1 One kilogram, as titanium oxide, of a titanium sulfate solution was taken out from a process of preparing titanium oxide by a sulfate method, to which 200 g of finely divided tungsten oxide. The mixture was sufficiently agitated and mixed, followed by addition of aqueous ammonia to neutralize and hydrolyze the titanium sulfate, permitting co-precipitation with the finely divided tungsten oxide. The precipitate was filtered, washed with water, dried at 100°C for 12 hours and sintered at a temperature of 500°C for 3 hours. The sintered product was milled in a sample mill and the resulting particles were suitably adjusted in size. Water was added to the particles and the mixture was kneaded, followed by extrusion molding in the form of a lattice by means of an extruder, heating from normal temperatures to 100°C for drying, and sintering at 500°C for 3 hours to obtain a catalyst having tungsten oxide thereon used for removal of nitrogen oxides. Example 2 One kilogram, as titanium oxide, of a titanium sulfate solution was taken out from a manufacture process of titanium oxide by a sulfate method, titanium sulfate was neutralized and hydrolyzed to give orthotitanic acid, which was subsequently filtered, washed with water, dispersed again in water and solated with barium chloride. The resulting sol was filtered, dried at 100°C for 12 hours, and sintered at a temperature of 500°C for 3 hours. The sintered product was milled in a sample mill and the particle size was suitably adjusted to obtain a powder carrier. 250 ml of a 10% methylamine solution containing 110 g of ammonium paratungstate was added to one kilogram of the powder carrier and kneaded, followed by extrusion molding in the form of a lattice by means of an extruder and heating for drying from normal temperatures to 100°C. The dried lattice was sintered at 500°C for 3 hours to obtain a catalyst having tungsten oxide, for removing nitrogen oxides. Example 3 An aqueous solution of 10 g of ammonium metavanadate and 25 g of oxalic acid in water was impregnated in 1 kg of the tungsten oxide-bearing catalyst obtained in Example 1, followed by drying at 100°C for 12 hours and sintering at 500°C for 3 hours, thereby obtaining a catalyst having tungsten oxide and vanadium oxide thereon. Example 4 Vanadium oxide was deposited on the tungsten oxide-bearing catalyst obtained in Example 2 in the same manner as in Example 3, thereby obtaining a catalyst for removal of nitrogen oxides Example 5 300 ml of a methylamine solution dissolving 120g of ammonium molybdenate was impregnated in 1 kg of the tungsten oxide-bearing catalyst obtained in Example 1, followed by drying at 100°C for 12 hours and sintering at 500°C for 3 hours to obtain a catalyst having tungsten oxide and molybdenum oxide thereon. Example 6 The general procedure of Example 5 was repeated using 1 kg of the tungsten oxide-bearing catalyst obtained in Example 2, thereby obtaining a catalyst having tungsten oxide and molybdenum oxide thereon. The catalysts for removal of nitrogen oxides obtained in the foregoing examples were each subjected to catalytic contact with a mixed gas of a composition comprising 200 ppm of nitrogen oxides, 200 ppm of ammonia, 10% of steam, 12% of carbon dioxide, 800 ppm of sulfur dioxide and the balance of nitrogen at a temperature of 380°C a space velocity of 5000 hr⁻¹. The rate of removal of nitrogen oxides (NOx) and the rate of oxidation of sulfur dioxide (SO₂) were measured. The results are shown in Table 1. The rate (%) of removal of nitrogen oxides and th rate (%) of oxidation of sulfur dioxide were calculated according to the following equations. Rate of removal of nitrogen oxide (%) = (concentration of NOx at the inlet of the catalyst bed - concentration of NOx at the outlet of the catalyst bed)/(concentration of NOx at the inlet of the catalyst bed) x 100 Rate of oxidation of sulfur dioxide (%) = (concentration of SO₂ at the inlet of the catalyst bed - concentration of SO₂ at the outlet of the catalyst bed)/ (concentrations of SO₂+SO₃ at the inlet of the catalyst bed ) x 100 Rate of Removal of NOx (%) Rate of Oxidation of SO₂ (%) Example 1 85 0.2 2 84 0.2 3 91 1.2 4 90 1.0 5 89 0.3 6 89 0.3 Example 7 A titanium sulfate solution obtained from a manufacture process of titanium oxide by a sulfate method was neutralized and hydrolyzed with aqueous ammonia to obtain orthotitanic acid. One kilogram, as titanium oxide, of the orthotitanic acid was taken out, to which 80 g of barium chloride dihydrate was added for solation, followed by sufficient agitation and mixing. Thereafter, 250 ml of a 10% methylamine solution containing 110 g of ammonium paratungstate was added to the mixture and sufficiently agitated and mixed, followed by drying at 100°C for 12 hours and sintering at a temperature of 500°C for 3 hours. The resulting sintered product was milled in a sample mill and the size was appropriately adjusted, after which a suitable amount of water was added and kneaded. Subsequently, the mixture was subjected to extrusion molding in the form of a lattice by means of an extruder, dried by heating from normal temperatures to 100°C and sintered at 500°C for 3 hours to obtain a sintered article. This article is useful as a catalyst for removal of nitrogen oxides. The X-ray spectrum of the sintered article is shown in FIG. 1, revealing that the peak is so low and broad that anatase crystals remain ungrown. The X-ray spectrum was measured using the X-ray diffraction analyzer, RAD-IIA. Example 8 One kilogram, calculated as titanium oxide, of orthotitanic acid as in Example 7 was taken out, to which 80 g of barium chloride dihydrate was added for solation, followed by sufficient agitation and mixing. Thereafter 300 ml of a methylamine solution containing 120 g of ammonium molybedenate, followed by repeating the procedure of Example 7 to obtain a sintered product. The product was subjected to extrusion molding in the same manner as in Example 7 to obtain a sintered article usable as a catalyst for removal of nitrogen oxides. Example 9 An aqueous solution containing 10 g of ammonium metavanadate and 25 g of oxalic acid was added to 1 kg of the sintered product obtained in Example 7, followed by sufficient kneading and extrusion molding in th form of a lattice by means of an extruder. The lattice molding was dried by heating from normal temperatures to 100°C and sintered at 500°C for 3 hours, thereby obtaining a catalyst for removal of nitrogen oxides. Example 10 The general procedure of Example 9 was repeated except that the sintered Product of Example 8 was used, thereby obtaining a catalyst for removal of nitrogen oxides. Example 11 Five percent by weight of manganese oxide was deposited on 1 kg of the sintered powder product obtained in Example 7, after which the procedure of Example 7 was repeated to obtain a sintered lattice article for use as a catalyst for removal of nitrogen oxides. Example 12 After deposition of 5 wt% of manganese oxide on 1 kg of the sintered powder product obtained in Example 8, a catalyst for removal of nitrogen oxides was obtained in the same manner as in Example 8. Example 13 After deposition of 5 wt% of manganese oxide on 1 kg of the sintered powder product obtained in Example 9, a catalyst for removal of nitrogen oxides was obtained in the same manner as in Example 9. Comparative Example 1 One kilogram, calculated as titanium oxide, of orthotitanic acid as used in Example 7 was taken out, to which 80 g of barium chloride dihydrate was added for solation, followed by sufficient agitation and mixing, drying at 100°C for 12 hours and sintering at 500°C for 3 hours. The sintered product was milled in a sample mill and the particle size was suitably adjusted. Thereafter, 250 ml of methylamine solution containing 110 g of ammonium paratungstate was added and kneaded, to which water was further added in suitable amounts and kneaded, followed by extrusion molding in the form of a lattice in the same manner as in Example 7 and sintering to obtain a catalyst for removal of nitrogen oxides. Comparative Example 2 An aqueous solution of 10g of ammonium ortho-vanadate and 25 g of oxalic acid dissolved in water was added to 1 kg of the sintered product obtained in Comparative Example 1, to which a suitable amount of water was further added and kneaded. The mixture was subjected to extrusion molding in the form of a lattice, dried at 100°C for 12 hours and sintered at 500°C for 3 hours to obtain a catalyst for removal of nitrogen oxides. Comparative Example 3 The general procedure of Example 7 was repeated except That orthotitanic acid was not solated with barium chloride, thereby obtaining a sintered article. Comparative Example 4 In Example 1, a manganese nitrate solution whose concentration was adjusted to 5 wt% as manganese oxide was added, instead of ammonium paratungstate, and kneaded, to which a suitable amount of water was added for further kneading. Thereafter, the mixture was subjected to extrusion molding in the form of a lattice and sintered in the same manner as in Example 12 to obtain a catalyst for removal of nitrogen oxides. Comparative Example 5 An aqueous solution of 10 g of ammonium metavanadate and 25 g of oxalic acid in water was added to 1 kg of the sintered product obtained in Comparative Example 4, to which a suitable amount of water was further added and kneaded. The mixture was subjected to extrusion moulding in the form of a lattice, dried at 100°C for 12 hours and sintered at 500°C for 3 hours to obtain a catalyst for removal of nitrogen oxides. Comparative Example 6 The general procedure of Example 8 was repeated except that orthotitanic acid was not solated with barium chloride to obtain a sintered article. The catalysts obtained in the above examples and comparative examples were each tested by contact with a mixed gas comprising 200 ppm of nitrogen oxides, 200 ppm of ammonia, 10% of steam, 12% of carbon dioxide, 800 ppm of sulfur dioxide and the balance of nitrogen at a temperature of 350°C at a space velocity of 5000 hr⁻¹ to measure a rate of removal of nitrogen oxides (NOx) and a rate of oxidation of sulfur dioxide (SO₂). The results are shown in Table 2. Rate of Removal of NOx (%) Rate of Oxidation of SO₂ (%) Example 7 89 0.1 8 88 0.1 9 95 0.6 10 95 0.6 11 90 0.1 12 90 0.1 13 96 0.6 Comp. Ex. 1 84 0.2 2 92 1.0 3 83 0.3 4 83 0.2 5 92 0.9 6 82 0.3 Example 14 A titanium sulfate solution obtained from a manufacture process of titanium oxide by a sulfate method was neutralized and hydrolyzed with aqueous ammonia to obtain orthotitanic acid. One kilogram, as titanium oxide, of the orthotitanic acid was taken out, to which 80 g of barium chloride dihydrate was added for solation, followed dried at 100°C for 12 hours and sintered at 500°C for 3 hours. The sintered product was milled in a sample mill, to which a 10% methylamine solution containing 110 g of ammonium paratungstate was added and kneaded. Thereafter, the mixture was subjected to extrusion molding in the form of a lattice, dried by heating from normal temperatures to 100°C and sintered at 500°C for 3 hours, thereby obtaining a sintered article. This was useful as a catalyst for removal of nitrogen oxides as will be described hereinafter. The X-ray spectrum of the sintered article is shown in FIG. 2, revealing that the peak is so low and wide that anatase crystals remain ungrown. The X-ray spectrum was measured using the X-ray diffraction analyzer, RAD-IIA, indicated before with the same measuring conditions as in Example 12. Example 15 300 ml of a methylamine solution containing 120 g of ammonium molybdenate (corresponding to 98 g of MoO₃) was added to the sintered product of the mixture of titanium oxide and ammonium paratungstate obtained in Example 14, followed by sintering in the same manner as in the second sintering in Example 14 to obtain a sintered product. This product in the form of a powder was subjected to extrusion molding and sintered to obtain a sintered article usable as a catalyst for removal of nitrogen oxides. EXAMPLE 16 An aqueous solution containing 10 g of ammonium metavanadate and 25 g of oxalic acid was added to 1 kg of the sintered product of the mixture of titanium oxide and ammonium paratungstate obtained in Example 14 and sufficiently kneaded. Thereafter, the mixture was subjected to extrusion molding in the form of a lattice by means of an extruder, followed by drying by heating from normal temperatures to 100°C, sintering at 500°C for 3 hours, and further extrusion molding, thereby obtaining a catalyst for removal of nitrogen oxides. Comparative Example 7 A titanium sulfate solution obtained from a manufacture process of titanium oxide by a sulfate method was thermally hydrolyzed to obtain metatitanic acid. One Kilogram, as titanium oxide, of the metatitanic acid was taken out to which 80 g of barium chloride dihydrate as a solating agent was added for solation, followed by sufficient agitation and mixing, drying at 100°C for 12 hours and sintering at a temperature of 500°C for 3 hours. One kilogram of the sintered product was milled in a sample mill with the size being adjusted, after which a 10% methylamine solution containing 110 g of ammonium paratungstate was added and kneaded. The mixture was subjected to extrusion molding in the form of a lattice by means of an extruder dried by healing from normal temperatures to 100°C and sintered at 500°C for 3 hours, thereby obtaining a sintered article. This article is usable as a catalyst for removal of nitrogen oxides. Comparative Example 8 The sintered product of the mixture of barium-containing titanium oxide and ammonium paratungstate obtained in Comparative Example 7 was mixed with 300 ml of a methylamine solution containing 120 g of ammonium molybedenate, followed by repeating the procedure of Comparative Example 7 to obtain a sintered product in the form of a powder. The powder was further subjected to extrusion molding and sintered to obtain a sintered article usable as a catalyst for removal of nitrogen oxides. Comparative Example 9 An aqueous solution containing ammonium metavanadate and 25 g of oxalic acid was added to the sintered product of the mixture of barium-containing titanium oxide and ammonium paratungstate obtained in Comparative Example 7, and sufficiently kneaded. Thereafter, the mixture was subjected to extrusion molding in the form of a lattice by means of an extruder, dried by heating from normal temperatures to 100°C and sintered at 500°C for 3 hours, followed by further extrusion molding to obtain a catalyst for removal of nitrogen oxides. Example 17 A titanium sulfate solution obtained from a manufacture process of titanium oxide by a sulfate method was neutralized and hydrolyzed with aqueous ammonia to obtain orthotitanic acid. One kilogram, as titanium oxide, of the orthotitanic acid was taken out, washed with water, dried at 100°C for 12 hours and sintered at 500°C for 3 hours. The sintered product was milled in a sample mill with the size being suitably adjusted, after which 300 ml of a methylamine soultion containing 120 g of ammonium molybdenate was added and kneaded. The mixture was subjected to extrusion molding in the form of a lattice by means of an extruder, dried by heating from normal temperatures to 100°C and sintered at 500°C for 3 hours to obtain a sintered article. The article was subjected to measurement of an X-ray spectrum under the same condition as in Example 14, from which it was confirmed that anatase crystals remained ungrown. Example 18 A titanium sulfate solution obtained from a manufacture process of titanium oxide by a sulfate method was neutralized and hydrolyzed with aqueous ammonia to obtain orthotitanic acid. One kilogram, as titanium oxide, of the orthotitanic acid was taken out, washed with water, dried at 100°C for 12 hours and sintered at 500°C for 3 hours. The sintered product was milled in a sample mill with the size being suitably adjusted, after which a 10% methylamine solution containing 110 g of ammonium paratungstate and 120 g of ammonium molybdenate was added and kneaded. Thereafter, the procedure of Example 14 was repeated to prepare a catalyst of a sintered article. The sintered article was subjected to measurement of an X-ray spectrum under the same conditions as in Example 14, from which it was confirmed that the anatase crystals were left as ungrown. Examples 19 An aqueous solution containing 10 g of ammonium metavanadate and 25 g of oxalic acid was added to 1 kg of the sintered product of the mixture of titanium oxide and ammonium molybdenate obtained in Example 17 and sufficiently kneaded. The mixture was subjected to extrusion molding in the form of a lattice by means of an extruder, dried by heating from normal temperatures to 100°C, sintered at 500°C for 3 hours and further subjected to extrusion molding to obtain a catalyst for removal of nitrogen oxides Example 20 A 10% methylamine solution containing 110 g of ammonium paratungstate was added to 1 kg of the sintered product of the mixture of titanium oxide and ammonium molybdenate obtained in Example 17 and sufficiently kneaded. The mixture was subjected to extrusion molding in the form of a lattice by means of an extruder, dried by heating from normal temperatures to 100°C, and sintered at 500°C for 3 hours to obtain a sintered article according to the invention. Example 21 An aqueous solution containing 10 g of ammonium metavanadate and 25 g of oxalic acid was added to 1 kg of the sintered product of the mixture of titanium oxide and ammonium paratungstate obtained in Example 18 and sufficiently kneaded. The mixture was subjected to extrusion molding in the form of a lattice by means of an extruder, dried by heating from normal temperatures to 100°C, sintered at 500°C for 3 hours and further subjected to extrusion molding to obtain a catalyst for removal of nitrogen oxides. Example 22 An aqueous solution of manganese nitrate having such a controlled concentration as to permit 5 wt% as manganese oxide was added to 1 kg of the sintered product of the mixture of titanium oxide and ammonium paratungstate obtained in Example 14, and sufficiently kneaded. The mixture was subjected to extrusion molding in the form of a lattice by means of an extruder, followed drying by heating from normal temperatures to 100°C, sintering at 500°C for 3 hours and further extrusion molding to obtain a catalyst for removal of nitrogen oxides. The catalysts obtained in the above examples and comparative examples were each tested by contact with a mixed gas comprising 100 ppm of nitrogen oxides, 100 ppm of ammonia, 10% of steam, 12% of carbon dioxide, 800 ppm of sulfur dioxide and the balance of nitrogen at a temperature of 360°C, a space velocity of 5000 hr⁻¹ to measure a rate of removal of nitrogen oxides (NOx) and a rate of oxidation of sulfur dioxide (SO₂). The results are shown in Table 3. Rate of Removal of NOx (%) Rate of Oxidation of SO₂ (%) Example 14 84.5 0.2 15 84.0 0.2 16 91.5 0.8 17 85.0 0.2 18 84.5 0.3 19 92.0 0.8 20 86.0 0.4 21 93.5 1.1 22 92.0 0.9 Comp. Ex. 7 80.5 0.4 8 80.0 0.4 9 89.5 1.5 As will be apparent from the above results, the catalysts obtained by the method of the invention have a high removal rate of nitrogen oxides with a low rate of oxidation of sulfur dioxide. Thus, the disadvantage based on the formation of sulfur trioxide during the removal of nitrogen oxides from mixed gases can be overcome.

Claims: A method for preparing a carrier for a catalyst used for removing nitrogen oxides which comprises sintering orthotitanic acid in coexistence of at least one compound selected from the group consisting of finely divided tungsten compounds and molybdenum compounds. The method for manufacturing a catalyst used for removing nitrogen oxides, wherein the product of Claim 1 is treated to carry active catalytic components for removing nitrogen oxides.

Inventors: IIDA, KOZO HIROSHIMA TECHN. INST., MITSUOKA, SHIGEAKI HIROSHIMA TECHN. INST., OBAYASHI, YOSHIAKI HIROSHIMA TECHN. INST., SERA, TOSHIKUNI HIROSHIMA TECHN. INST., SETO, TOURU HIROSHIMA TECHN. INST., SUZUMURA, HIROSHI HIROSHIMA TECHN. INST.
Applicants: MITSUBISHI JUKOGYO KABUSHIKI KAISHA
Application year: 1986
Main classification: B01D53
IPC codes: B01D53, B01J23