Catalyst for catalytic reduction of nitrogen oxide

Abstract: A catalyst for catalytic reduction of nitrogen oxide using a hydrocarbon and/or an oxygen-containing organic compound as a reducing agent, which comprises a perovskite type compound oxide represented by formula (I), (IV), or (V): A¹ x B¹ 1-x C¹O₃ (I) wherein A¹ represents at least one element selected from the group consisting of La, Y, Ce, Pr, Nd, Pm, Sm, Eu, and Gd; B¹ represents at least one element selected from the group consisting of Na, K, Bi, Th, Ba, Sr, Ca, Mg, Pb, Zn, and Ag; C¹ represents at least one element selected from the group consisting of Mn, Co, Fe, Ni, Cr, Cu, V, Mo, W, Ta, Li, Ti, Zr, Nb, Pd, Rh, Ru, and Pt; and x is a number of from 0 to 1, A⁴ 2-x B³ x C⁴O₄ (IV) wherein A⁴ represents at least one element selected from the group consisting of Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, and Bi; B³ represents at least one element selected from the group consisting of Ba, Sr, Ca, Mg, Pb, Zn, and Ag; C⁴ represents at least one element selected from the group consisting of Mn, Co, Fe, Ni, Cr, Cu, V, Mo, W, Ti, Zr, Nb, Pd, Rh, Ru, and Pt; and x is a number of from 0 to 1, A⁵ x B³ 3-x C⁴₃O₇ (V) wherein A⁵ represents at least one element selected from the group consisting of La, Y, Ce, Pr, Nd, Sm, Eu, Gd; B³ represents at least one element selected from the group consisting of Ba, Sr, Ca, Mg, Pb, Zn, and Ag; C⁴ represents at least one element selected from the group consisting of Mn, Co, Fe, Ni, Cr, Cu, V, Mo, W, Ti, Zr, Nb, Pd, Rh, Ru, and Pt; and x is a number of from 0 to 1, said perovskite compound being supported on a solid acid carrier. The catalyst selectively catalyzes a reaction of nitrogen oxide with the reducing agent even in the presence of oxygen so that nitrogen oxide in emissions can be reduced efficiently without requiring a large quantity of the reducing agent.

Description: FIELD OF THE INVENTION This invention relates to a catalyst for catalytic reduction of nitrogen oxide. More particularly, it relates to a catalyst for reduction of nitrogen oxide using a hydrocarbon and/or an oxygen-containing organic compound as a reducing agent, which is suitable for reducing and removing harmful nitrogen oxide present in emissions from factories, automobiles, etc. BACKGROUND OF THE INVENTION Nitrogen oxide present in exhaust or emissions have been removed by, for example, a method in which the nitrogen oxide is further oxidized and then absorbed in an alkali or a method comprising reducing it to nitrogen by using ammonia, hydrogen, carbon monoxide, hydrocarbons, etc. as a reducing agent. These conventional methods have their own disadvantages. That is, the former method requires a means for handling the alkaline waste liquid to prevent environmental pollution. Of the latter method, the method of using ammonia as a reducing agent involves the problem that ammonia reacts with sulfur oxides in the emissions to form salts, resulting in a reduction in catalyst activity. When in using hydrogen, carbon monoxide, or a hydrocarbon as a reducing agent, the reducing agent preferentially undergoes reaction with oxygen because oxygen is present in a higher concentration than nitrogen oxide in emissions. This means that substantial reduction of nitrogen oxide requires a large quantity of the reducing agent. It has recently been proposed to catalytically decompose nitrogen oxide in the absence of a reducing agent. However, known catalysts for direct decomposition of nitrogen oxide have not yet been put to practical use due to their low decomposing activity. On the other hand, H type zeolite, Cu ion-exchanged ZSM-5, etc. have also been proposed as a catalyst for catalytic reduction of nitrogen oxide using a hydrocarbon or an oxygen-containing organic compound as a reducing agent. In particular, H type ZSM-5 (SiO 2 /Aℓ 2 O 3 molar ratio = 30 to 40) are regarded optimal. However, even the H type ZSM-5 is not deemed to have sufficient reducing activity, and it has been demanded to develop a highly active catalyst for catalytic reduction of nitrogen oxide. JP-A-1 030 649 discloses a nitrogen oxide decomposition catalyst for treating exhaust gases. The catalyst is composed of a platinum group metal supported by a perovskite composite oxide. The perovskite composite oxide may be supported in turn by a pelletised or honeycomb-form of alumina or silica with an inorganic binder such as colloidal alumina or silica. FR-A-2 343 505 discloses a catalyst comprising a perovskite and alumina supported on a refractory oxide carrier such as alumina. US-A-3 926 854 discloses mixed oxide compounds of perovskite-type formula as an oxidation catalyst for carbon monoxide by reaction with nitric oxide in exhaust gases. The catalyst is supported on e.g. a ceramic refractory, glass or high-melting metal support. EP-A-0 089 199 discloses a family of perovskite oxides which may be used to convert exhaust gases. These are supported on an alumina carrier. SUMMARY OF THE INVENTION An object of the present invention is to provide a catalyst for catalytic reduction of nitrogen oxide using a hydrocarbon or an oxygen-containing organic compound as a reducing agent, which selectively catalyzes a reaction of nitrogen oxide with the reducing agent even in the presence of oxygen so that nitrogen oxide in emissions can be reduced efficiently without requiring a large quantity of the reducing agent. According to one aspect, the present invention provides a catalyst for catalytically reducing nitrogen oxide using a hydrocarbon and/or an oxygen-containing organic compound as a reducing agent, which comprises a zeolite type solid acid carrier supporting 30-50 parts by weight, based on 100 parts by weight of the carrier, of a perovskite type compound oxide of the formula LaCoO 3 . Preferably the zeolite type solid acid carrier is an acid type mordenite. It is preferred that the zeolite type solid acid carrier includes at least one element selected from Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Y, Zr, Nb, Ru, Rh, Pd, Pt, Ag, Sn, La, Nd, Ce, Pb, Mg, Ca, Sr, Ba, and Bi. DETAILED DESCRIPTION OF THE INVENTION The terminology "solid acid carrier" as used in the present invention means a carrier which is solid and acidic in a temperature at which the catalyst is used. Solid acidity can be confirmed by a temperature rise release method using ammonia or an in situ FTIR (Fourier transform infrared spectrophotometry) method using ammonia or pyridine. The solid acid carrier which is used in the present invention is a zeolite type solid acid carrier. The zeolite type solid acid carriers can be obtained by treating heat-resistant zeolite, such as Na-mordenite, Na-ZSM-5, Na-USY (USY: ultrastable Y type zeolite), or metallosilicates (zeolite whose aluminum is partly or wholly replaced with another metallic element, especially iron, gallium, zinc, lanthanum, copper, molybdenum, chromium, germanium, titanium, or boron), with an aqueous solution of an ammonium salt (e.g., ammonium sulfate) or an acid (e.g., sulfuric acid) to exchange a part of or the whole of the alkali metal of the zeolite for an ammonium ion or a hydrogen ion. In the case where the alkali metal is exchanged for an ammonium ion, the product finally requires calcining. Examples of the zeolite type solid acid carrier include acid type mordenite represented by formula (a) shown below, which is obtained by an acid treatment of mordenite type zeolite: M 2 ((AℓO 2 ) 2±r (SiO 2 ) 10 )·zH 2 O wherein M represents an alkali metal ion; and r is a number subject to variation depending on the conditions of zeolite synthesis, with the SiO 2 /Aℓ 2 O 3 molar ratio ranging from 13 to 20 and the SiO 2 /H 2 O molar ratio ranging from 25 to 200, and zeolite represented by formula (b) shown below, which is obtained by exchanging a part of or the whole of ion M of zeolite for a titanium ion (Ti 4+ ), a zirconium ion (Zr 4+ ), or a tin ion (Sn 4+ ): M' A ((AℓO 2 ) p (SiO 2 ) q )·z'H 2 O wherein M' represents an alkali metal ion, an alkaline earth metal ion, or a hydrogen ion; nA=P (wherein n is a valence number of ion M'); and q/p is 5 or more. The catalyst described above can be prepared by the following processes (A) to (C). Process (A): A water-soluble salt (e.g., nitrate) of La and Co or an alcohol solution of an alkoxide of the above-mentioned element is poured into a slurry of a solid acid carrier, and the slurry is neutralized or hydrolyzed to form a precipitate. Otherwise, the slurry is spray dried or freeze-dried to make a precursor of a perovskite compound, such as a compound hydroxide of these metals, be supported on the solid acid carrier. The resulting solid is worked up by repeatedly subjecting to filtration, washing with water, and repulping, and then dried and calcined. Process (B): A solid acid carrier and a separately prepared perovskite compound are thoroughly wet ground by means of a planetary mill, etc. Process (C): A precursor of a solid acid carrier, such as a water-soluble salt or a hydroxide, and a water-soluble salt (e.g., nitrate) of La and Co, or an alcohol solution of an alkoxide of the above-mentioned element are uniformly mixed, and the resulting homogeneous solution is neutralized or hydrolyzed to form a precipitate. The precipitate is worked up by repeatedly subjecting to filtration, washing with water, and repulping, and then dried and calcined. In the above-mentioned embodiments, at least one element selected from the group consisting of Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Y, Zr, Nb, Ru, Rh, Pd, Pt, Ag, Sn, La, Nd, Ce, Pb, Mg, Ca, Sr, Ba, and Bi can be supported on the solid acid carrier by any of known techniques, such as an ion-exchange method, a precipitation method, and a mixing method. Of these methods, an ion-exchange method is preferred. For example, a part of or the whole of ion M or M' of zeolite represented by formula (a) or (b) can be displaced with the above-enumerated element(s) by a known ion-exchange method. The catalysts of such embodiments can be prepared, for example, by the following processes (D) or (F). Process (D): Zeolite, for example, the zeolite represented by formula (a) or (b) is poured into an aqueous solution of a water-soluble salt (e.g., nitrate) of one or more of Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Y, Zr, Nb, Ru, Rh, Pd, Ag, Sn, La, Nd, Ce, Pb, Mg, Ca, Sr, Ba, and Bi to thereby exchange M 2 of formula (a) or M' A of formula (b) for the metal ion(s) or the metal ion(s) and a hydrogen ion. Where a desired ion-exchange rate is not reached through a single ion-exchange operation, the above-mentioned procedure is repeated. The slurry is repeatedly subjected to filtration, washing, and repulping, and finally dried. If desired, the product is calcined. Into a slurry of the thus prepared carrier are simultaneously poured a water-soluble salt (e.g., nitrate) of La and Co or an alcohol solution of an alkoxide of the above-mentioned element and a neutralizing agent (e.g., aqueous ammonia) or a precipitating medium to precipitate a precursor of a perovskite compound on the carrier. The perovskite compound precursor-on-carrier is repeatedly subjected to filtration, washing, and repulping and finally dried and calcined. Process (E): The solid acid carrier obtained by process (D) and a separately prepared perovskite compound are thoroughly wet ground in a planetary mill, etc. and dried. If desired, the dried product is calcined. In carrying out the above-described processes, it is preferable to keep the temperature of forming a perovskite compound low. With the temperature being controlled low, a possible reaction between the solid acid carrier and the elements constituting the perovskite compound can be inhibited so as to prevent denaturation of the solid acid carrier. Further, the lower the temperature, the greater the specific surface area of the resulting perovskite compound. As a result, a reduction in catalyst activity due to a reduction in yield of the perovskite compound can be avoided. Accordingly, when a solid acid carrier showing high reactivity with the elements constituting a perovskite compound, such as Aℓ 2 O 3 or TiO 2 , is used, process (C) is not suitable because, in process (C), the elements constituting a solid acid carrier and those constituting a perovskite compound are mixed in high homogeneity. In general, process (A) is recommended. Catalysts having fairly high activity can also be obtained by process (B). The amount of the perovskite compound to be supported on the solid acid carrier preferably ranges from 30 to 50% by weight based on the carrier. The catalyst according to the present invention may be shaped into various shapes, such as a honeycomb shape and a spherical shape, by any of well-known shaping methods. If desired, appropriate additives, such as shaping assistants, reinforcements, inorganic fibers, and organic binders, may be used on shaping. The catalyst may be coated on a previously molded base by, for example, a wash coat method. Preparation of the catalyst is not limited to the above-mentioned methods, and any of known catalyst preparation techniques may be applied. The hydrocarbons which can be used as a reducing agent in the present invention include gases, such as methane, ethane, and butylene; and liquids, such as single component hydrocarbons, e.g., pentane, hexane, octane, heptane, benzene, toluene, and xylene, and petroleum hydrocarbons, e.g., gasoline, kerosine, gas oil, and heavy oil. Preferred of them are lower alkynes, e.g., acetylene, methylacetylene, and 1-butyne; lower alkenes, e.g., ethylene, propylene, isobutylene, 1-butene, and 2-butene; lower dienes, e.g., butadiene and isoprene; and lower alkanes, e.g., propane and butane. The amount of the hydrocarbon to be used, though varying depending on the kind, is preferably from about 0.1 to 2 mols per mol of nitrogen oxide. If the hydrocarbon molar ratio is less than 0.1, sufficient reducing activity cannot be obtained. If it exceeds 2, the amount of the hydrocarbon which is withdrawn as being unreacted increases, needing any post treatment for recovery of the unreacted hydrocarbon.. The terminology "oxygen-containing organic compound" as used herein for a reducing agent means an organic compound having an oxygen element in the molecule thereof. Specific examples of the oxygen-containing organic compound include alcohols, e.g., methyl alcohol, ethyl alcohol, propyl alcohol, and octyl alcohol; ethers, e.g., dimethyl ether, diethyl ether, and dipropyl ether; esters, e.g., methyl acetate, ethyl acetate, and fats and oils; and ketones, e.g., acetone and methyl ethyl ketone; with lower alcohols, e.g., methyl alcohol and ethyl alcohol, being preferred. The above-mentioned hydrocarbons and oxygen-containing organic compounds may be used either singly or in combination of two or more thereof. One or more of the hydrocarbons and one or more of the oxygen-containing organic compounds may be used in combination. In addition, uncombusted matters or incomplete combustion products of fuels, etc. present in emissions, i.e., hydrocarbons and particulate matters, are also effective as a reducing agent and, accordingly, included in the term "hydrocarbons" as used in the present invention. In other words, the catalyst of the present invention is deemed useful for removal or reduction of the hydrocarbons or particulate matters of emissions from cars, factories, etc. The temperature at which the above-mentioned reducing agent selectively reduces nitrogen oxide ascends in the order of oxygen-containing organic compounds, alkynes, alkenes, aromatic hydrocarbons, and alkanes. The greater the carbon atom number of hydrocarbons, the lower the effective temperature. A suitable temperature for the catalyst of the present invention to show catalysis in reduction of nitrogen oxide usually ranges from 100 to 800°C, and preferably from 200 to 600°C, though varying depending on the kind of the reducing agent or catalyst species used. Within the above-recited temperature range, exhaust gas to be treated is preferably made to flow at a space velocity (SV) of from 500 to 100000 hr -1 . The present invention is now illustrated in greater detail with reference to Examples, but it should be understood that the present invention is not deemed to be limited thereto. All the parts, percents, and ratios are by weight unless otherwise indicated. EXAMPLE A-1 (Reference) In 500 mℓ of water were dissolved 101.05 g of lanthanum nitrate hexahydrate (La(NO 3 ) 2 ·6H 2 O), 28.60 g of manganese acetate tetrahydrate (Mn(Ac) 2 ·4H 2 O; Ac is CH 3 COO, hereinafter the same), 74.08 g of strontium nitrate (Sr(NO 3 ) 2 ), and 135.83 g of cobalt nitrate hexahydrate (Co(NO 3 ) 2 ·6H 2 O). The resulting aqueous solution was adjusted to a pH of 10 by addition of a 121 g/ℓ aqueous solution of sodium hydroxide with thorough stirring. After a precipitate was formed, the stirring was further continued for an additional period of 18 hours for maturing. The mixture was repeatedly filtered, washed with water, and repulped until the filtrate had substantially the same conductivity as that of repulping water. The resulting filter cake was dried at 120°C for 18 hours and then calcined at 700°C for 3 hours. The X-ray diffractometry of the resulting calcined product revealed formation of a perovskite crystal phase. The calcined product had a specific surface area of 23.7 m 2 /g as measured by a BET method (hereinafter the same) (La 0.4 Sr 0.6 Co 0.8 Mn 0.2 O 3 ). Separately, metatitanic acid (TiO 2 ·H 2 O) obtained from a sulfuric acid process titanium oxide step was calcined at 600°C for 3 hours to prepare activated titanium oxide (specific surface area: 104.2 m 2 /g). Thirty grams of the perovskite compound were mixed with 100 g of the activated titanium oxide, and 100 g of water was added thereto. The mixture was ground in a planetary mill for 30 minutes, and the viscosity of the resulting slurry was adjusted with water to prepare a wash coating slurry. The slurry was coated on a honeycomb having a pitch of 1.25 mm (a product of Cordierite Co.; the same honeycomb was used in all the following examples) to obtain a catalyst-on-honeycomb (designated sample A-1) having 0.116 g of the catalyst per cm 3 of the honeycomb. EXAMPLE A-4 A perovskite compound (LaCoO 3 ) was obtained in the same manner as in Example A-1, except for using 88.07 g of lanthanum nitrate hexahydrate and 59.19 g of cobalt nitrate hexahydrate (Co(NO 3 ) 2 ·6H 2 O) and conducting calcination at 800°C for 3 hours. The resulting perovskite compound had a specific surface area of 17.4 m 2 /g. To a mixture of 30 g of the perovskite compound and 100 g of H type mordenite ("HM-23" produced by Nippon Kagaku Co., Ltd.) was added 100 g of water to prepare a slurry. The slurry was coated on a honeycomb to prepare sample A-4 having 0.113 g of the catalyst per cm 3 of the honeycomb. Example A-21 (Comparative) Sample A-21 was prepared in the same manner as in Example A-4, except for using 1 g of the perovskite compound (LaCoO 3 ) and 100 g H type mordenite ("HM-23"). The sample contained 0.929 g of the catalyst per cm 3 of the honeycomb. EXAMPLE A-22 (Comparative) Sample A-22 was prepared in the same manner as in Example A-4, except for using 10 g of the perovskite compound (LaCoO 3 ) and 100 g of H type mordenite ("HM-23). The sample contained 0.948 g of the catalyst per cm 3 of the honeycomb. EXAMPLE A-23 Sample A-23 was prepared in the same manner as in Example A-4, except for using 50 g of the perovskite compound (LaCoO 3 ) and 100 g of H type mordenite ("HM-23). The sample contained 0.109 g of the catalyst per cm 3 of the honeycomb. EXAMPLE A-24 (Comparative) Sample A-24 was prepared in the same manner as in Example A-4, except for using 50 g of the perovskite compound (LaCoO 3 ) and 50 g of the H type mordenite ("HM-23"). The sample contained 0.122 g of the catalyst per cm 3 of the honeycomb. Each of samples A-4, A-21, A-22, A-23 and A-24 was evaluated for its catalysis in reduction of nitrogen oxide as follows. A nitrogen oxide-containing gas was catalytically reduced with a reducing agent shown in Table 1 below in the presence of each sample. A conversion of nitrogen monoxide to N 2 was calculated from N 2 amount as measured by gas chromatography. Test Conditions: 1) Gas Composition: NO: 1% by volume O 2 : 10% by volume Reducing agent: 1% by volume He: balance 2) Space Velocity: 20000 hr -1 3) Reaction Temperature: 300°C, 400°C, 500°C, or 600°C The results obtained are shown in Table 1. Catalyst Reducing Agent Nitrogen Conversion (%) 300°C 400°C 500°C 600°C A-1 (Reference) propane 87.1 96.9 88.6 64.4 propylene 98.4 95.2 78.6 41.5 1,3-butadiene 95.6 92.0 78.0 32.3 ethanol 96.1 90.3 45.8 26.6 A-4 propane 96.1 98.3 92.5 72.0 isobutane 92.6 97.7 90.0 68.4 propylene 100 99.2 84.6 73.8 trans-2-butene 97.3 94.1 80.2 61.5 A-21 (Comp.) propylene 71.5 78.9 67.3 42.0 A-22 (Comp.) propylene 92.6 86.8 77.2 63.4 A-23 propylene 99.2 98.4 87.6 69.8 A-24 (Comp.) propylene 94.1 88.5 70.0 44.9 As is apparent from Table 1, each of the catalysts according to the present invention (samples A-4 and A-23) achieves a high conversion of nitrogen oxide to nitrogen, whereas the comparative catalysts (sample A-21, A-22 and A-24) exhibit a lower catalyzing activity. As described and demonstrated above, the catalyst according to the present invention efficiently catalyzes reduction of nitrogen oxide in emissions in the presence of oxygen. While the invention has been described in detail and with reference to specific examples thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the scope thereof.

Claims: A catalyst for catalytically reducing nitrogen oxide using a hydrocarbon and/or an oxygen-containing organic compound as a reducing agent, which comprises a zeolite type solid acid carrier supporting 30-50 parts by weight, based on 100 parts by weight of the carrier, of a perovskite type compound oxide of the formula LaCoO 3 . A catalyst according to Claim 1, wherein the zeolite type solid acid carrier is an acid type mordenite. A catalyst according to Claim 1, wherein the zeolite type solid acid carrier includes at least one element selected from Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Y, Zr, Nb, Ru, Rh, Pd, Pt, Ag, Sn, La, Nd, Ce, Pb, Mg, Ca, Sr, Ba, and Bi.

Inventors: HAMADA HIDEAKI, ITO TAKEHIKO, KAWATSUKI MASAAKI, KINTAICHI YOSHIAKI, KITAZUME AKIHIRO, MIYAMOTO KATSUMI, NAKATSUJI TADAO, SASAKI MOTOI, SHIMIZU HIROMITSU, SUGANUMA FUJIO, TABATA MITSUNORI, TSUCHIDA HIROSHI, YASUKAWA RITSU, HAMADA, HIDEAKI, ITO, TAKEHIKO, KAWATSUKI, MASAAKI, KINTAICHI, YOSHIAKI, KITAZUME, AKIHIRO, MIYAMOTO, KATSUMI, NAKATSUJI, TADAO, SASAKI, MOTOI, SHIMIZU, HIROMITSU, SUGANUMA, FUJIO, TABATA, MITSUNORI, TSUCHIDA, HIROSHI, YASUKAWA, RITSU, HAMADA, HIDEAKI, C/O NATIONAL CHEMICAL, Ito, Takehiko, c/o National Chemical Lab. for Ind., KINTAICHI, YOSHIAKI, NAT. CHEMICAL LAB. FOR IND., Nakatsuji, Tadao, c/o Sakai Chemical Ind. Co.,, Sasaki, Motoi, c/o National Chemical Lab. for Ind., Shimizu, Hiromitsu, c/o Sakai Chemical Ind. Co.,, Yasukawa, Ritsu, c/o Sakai Chemical Ind. Co.,
Applicants: AGENCY IND SCIENCE TECHN, COSMO OIL CO LTD, PETROLEUM ENERGY CENTER FOUND, SAKAI CHEMICAL INDUSTRY CO, AGENCY OF INDUSTRIAL SCIENCE AND TECHNOLOGY, COSMO OIL COMPANY, LTD, PETROLEUM ENERGY CENTER, SAKAI CHEMICAL INDUSTRY CO., LTD.,
Application year: 1992
Main classification: B01D53
IPC codes: B01J23, B01D53