Use of a catalyst for the elimination of nitrogen oxides from an exhaust gas

Abstract: The catalyst for purifying an exhaust gas has at least gallium and a zeolite and an exhaust gas purifying method using the catalyst. Nitrogen oxides in the exhaust gas are reduced and eliminated by the catalyst in an oxidative atmosphere at a reaction temperature of 200 to 800°C and under the presence of a hydrocarbon at the total THC concentration/NO x concentration of from 0.5 to 50.

Description: BACKGROUND OF THE INVENTION 1. FIELD OF THE INVENTION The present invention concerns the use of an exhaust gas purifying catalyst which decomposes, into non-toxic gases, nitrogen oxides discharged from mobile internal combustion engines used, for example, in diesel cars, stationary internal combustion engines used, for example, in cogeneration systems and various industrial furnaces such as boilers. 2. DESCRIPTION OF THE RELATED ART Generally, exhaust gases discharged from automobiles, stationary internal combustion engines and various industrial furnaces contain a great amount of nitrogen oxides represented by NO and NO 2 (NO x ). It is said that such NO x not only causes photochemical smog but also induces disease in man's respiratory organs. As a method of decreasing NO x , an exhaust gas processing technique of a so-called ternary catalyst system is established that eliminates NO x by reduction in an exhaust gas of low oxygen content such as that from gasoline cars by using a reducing agent such as carbon monoxide or hydrocarbon. On the other hand, in the case of an exhaust gas containing a great amount of oxygen such as that discharged from large-scaled stationary exhaust emission sources such as boilers, a selective NO x reduction process for decreasing the amount of NO x by external addition of ammonia is now under actual operation, which produces some effect. However, the former method is applicable only to an exhaust gas from a gasoline engine in which the oxygen concentration is extremely low whereas the latter method is difficult to use in small-sized stationary exhaust emission sources or mobile exhaust emission sources from a standpoint of handling because ammonia is used. In view of the above, various methods have been studied for using hydrogen, carbon monoxide or various hydrocarbons as reducing agents other than ammonia but most of them have the drawback that they are a non-selective catalytic reduction process which can eliminate nitrogen oxides only after oxygen in the exhaust gas has been consumed completely. Although the following methods have been proposed so far as a novel selective catalytic reduction process capable of overcoming such a drawback (a method of selectively reducing and eliminating nitrogen oxides even under the coexistence of oxygen), none of them can provide quite satisfactory results. That is, Japanese Patent Laid-Open Hei JP-A-2-149317 proposes a method of using (1) a catalyst comprising a hydrogen type mordenite or clinoptilolite or (2) a catalyst comprising a hydrogen type mordenite or clinoptilolite carrying a metal such as Cu, Cr, Mn, Fe and Ni, and bringing an exhaust smoke containing oxygen resulting from combustion of various fuels into contact with the above-mentioned catalyst under the coexistence of an organic compound thereby eliminating nitrogen oxides in the exhaust smoke. According to this method, a denitrating ratio of 30 to 60% is obtained under the conditions of a reaction temperature of 300 to 600°C and a gas hourly space velocity (GHSV) of 1200 h -1 , but a denitrating effect under a high GHSV condition, i.e., a condition approximate to that of practical use is not clear. Further, the literatures gives no description of the aging of the catalytic activity and the life of the catalyst is not clear. Further, since the catalyst is evaluated for a pseudo exhaust gas containing no SO x , the resistance of the catalyst to SO x is uncertain. Japanese Patent Laid-Open Hei JP-A-1-130735 proposes a method of using a catalyst in which a zeolite ion-exchanged with a transition metal (Cu, Co, Ni, Fe, Mg, Mn or the like) is carried on a refractory support, and capable of purifying nitrogen oxides even in an oxidative atmosphere. This is a method of purifying nitrogen oxides in an exhaust gas from a gasoline engine at high efficiency even in a lean air/fuel ratio region, in which the oxygen concentration in the exhaust gas is only about 3% at the highest. Accordingly, it is uncertain whether or not nitrogen oxides can be selectively denitrated by reduction also in an exhaust gas such as that from a diesel engine in which the oxygen concentration is from 5 to 10%. Also in the examples, the NO x reduction tends to be lowered greatly along with an increase in the oxygen concentration. Japanese Patent Laid-Open Sho 63-283727 proposes a method of using a catalyst in which a metal such as Cu, V, Mn, Fe or Cr is carried on a hydrophobic zeolite with a SiO 2 /Al 2 O 3 ratio of 15 or more and decreasing nitrogen oxides in an oxygen containing exhaust gas from an internal combustion engine under the presence of carbon monoxide and one kind or more of hydrocarbons. In this method, the denitrating ratio is decreased to as low a value as 4 to 26% in the case of using a zeolite catalyst carrying a metal other than copper. On the other hand, in the case of using a copper-zeolite catalyst, there is the problem that the copper ingredient is readily poisoned by SO x though a relatively high activity can be obtained. The oxygen concentration in the exhaust gas shown in the examples is 1.6% and it is uncertain whether or not nitrogen oxides can also be reduced selectively for denitration if the oxygen concentration is higher, for example, as in an exhaust gas from a diesel engine. Japanese Patent Laid-Open sho 63-100919 proposes a method of using a catalyst in which copper is carried on a porous support such as of alumina, silica or zeolite and eliminating nitrogen oxides in an exhaust gas containing oxygen under the presence of a hydrocarbon. In this method, the denitrating ratio is from 10 to 25% and no high denitrating activity is obtainable. Further, since the catalyst contains copper, there is the problem that the copper ingredient is readily poisoned by SO x . Further, the oxygen concentration in the exhaust gas shown in the examples is 2.1% and it is uncertain whether or not nitrogen oxides can also be reduced selectively for denitration if the oxygen concentration is higher. EP-A-0 060 740 discloses a supported catalyst consisting of cerium, iron, gallium and/or yttrium, platinum and/or palladium, and iridium and/or rhodium. Tile catalysts do not comprise any alkaline metal elements or alkaline earth metal elements. SUMMARY OF THE INVENTION The present invention provides the use of a catalyst as defined in claim 1. The exhaust gas purifying catalyst used according to the invention is an catalyst for reducing and eliminating nitrogen oxides in the exhaust gas under the presence of a hydrocarbon in an oxidative atmosphere, the catalyst comprising (1) gallium as a main catalyst, (2) at least one of alkali metal elements and alkaline earth metal elements as a promoter and (3) a zeolite. The oxidative atmosphere means herein an atmosphere containing an amount of oxygen in excess of that required for completely oxidizing carbon monoxide, hydrogen and hydrocarbon contained in an exhaust gas and a reducing substance of a hydrocarbon added as required in this process and converting them into H 2 O and CO 2 . As the Ga source for the gallium (Ga), any of compounds convertible into oxides upon preparation of the catalyst or during use thereof for reaction may be used. Such a compound includes Ga-nitrate, -sulfate, -oxide, -halide, -carbonate, -hydroxide and -organic acid salt. The content of gallium in the catalyst is usually from 0.01 to 10% by weight, preferably, 0.1 to 5% by weight of the entire catalyst when converted to an oxide. If the content is less than 0.01% by weight, enough catalytic activity is not obtained. On the other hand, if the content exceeds 10% by weight, no further improvement to the catalyst activity can be observed. While there is no particular restriction on the kind of the zeolite, it is preferred to use a zeolite with a Si/Al ratio (atom ratio) of 5 or more when it is used for the reaction or a zeolite with a Si/(Al+M) ratio (atom ratio) of 5 or more where Al atoms are partially or wholly replaced with an element (M) such as B, P, Ti, for example, MFI or MEL type zeolite such as ZSM-5, ZSM-8, ZSM-11 and Silicalite, or MTT, FER or OFL type zeolite. If the Si/Al or Si/(Al+M) ratio (atom ratio) is less than 5, the heat resistance of the zeolite is relatively low, so that there is a probability that the life of the catalyst will be shortened. As the alkali metal elements (Li, Na, K or the like) and the alkaline earth metal elements (Mg, Ca, Sr, Ba or the like), any of compounds convertible into oxides upon preparation of the catalyst or during use thereof for reaction can be used. Such a compound can include, for example, nitrate, sulfate, oxide, halide, carbonate, hydroxide and organic acid salt of these metal elements. The content of these promoters in the catalyst is usually from 0.01 to 1% by weight and, preferably, from 0.05 to 0.5% by weight of the entire catalyst when converted to an oxide. If the content is less than 0.01% by weight or more than 1 by weight, as mentioned, enough catalytic activity is no longer obtainable. The Ga and promoters may be present in an optional form in the catalyst and take the following forms. For instance, they may be contained as constituents of a zeolite (for example, ion-exchanging cations, substituent atoms in the framework, or various forms due to modification by processing upon preparation, pre-treatment and during reaction of the catalyst). Alternatively, the Ga and promoters may be incorporated into the catalyst, with being carried on a support comprising the zeolite containing Ga and the promoters described above, a zeolite containing no Ga or a mixture thereof, for example, by means of ion exchange, impregnation or gas phase deposition or they may be incorporated in a form physically mixed with the above-mentioned zeolite. Further, various metals may be carried on them. They are, preferably, formulated as a gallometallo silicate zeolite, gallosilicate zeolite or a zeolite modified therefrom. The catalyst may be in an optional shape, for example, of pellet, plate, column, honey comb or lattice. The catalyst used according to the present invention can be prepared, for example, by using compounds containing the Ga and promoters to a zeolite by means of ion-exchange, impregnation, physical mixing or gas phase deposition. Alternatively, compounds of the Ga and promoter may be incorporated into a gel upon synthesis of the zeolite simultaneously. Further, a catalyst preparation method of coating a catalyst on a lattice-like support such as that of cordierite, mullite or aluminum or on a substrate made of metal gauze may be adopted. The use of the catalyst according to the present invention has a feature of bringing an exhaust gas under the presence of a hydrocarbon in an oxidative atmosphere into contact with the catalyst and eliminating nitrogen oxides in the exhaust gas by reducing them into N 2 and H 2 O. The hydrocarbon described above may be a hydrocarbon remaining in the exhaust gas, but it is preferred to add a hydrocarbon from the outside if the amount of the hydrocarbon is less than the amount required for causing the denitrating reaction or if it is not contained at all in the exhaust gas. There is no particular restriction on the kind of the hydrocarbon to be added for this purpose and they may be methane, LPG, gasoline, gas oil, kerosene, A heavy oil or the like. The amount of the hydrocarbon present in the exhaust gas is from 0.5 to 50, preferably, 1 to 20, as indicated by the THC concentration/NO x concentration. The THC (Total Hydro-Carbon) concentration means a concentration of a hydrocarbon as converted into that of methane. For instance, if the NO x concentration is 1000 ppm, the THC concentration is from 0.05 to 5.0%. If the amount of the hydrocarbon present is lower than the above-mentioned lower limit, no denitrating effect appears. If it is higher than the above-mentioned upper limit, it is not preferred because the reduction of the economical performance of the entire system or abnormal heat generation in the catalyst bed caused by the combustion heat of the hydrocarbon, although the denitrating ratio is increased. The catalytic reaction temperature is set to 200 to 800°C, and preferably, 300 to 600°C. Usually, the denitrating ratio is increased as the temperature is higher, but, if it exceeds 800°C, undesirable degradation of the catalyst occurs and, on the other hand, the denitrating ratio is lowered if the temperature is lower than 200°C. The gas hourly space velocity (GHSV) is set usually to 2,000 to 200,000 h -1 , and preferably, 5,000 to 100,000 h -1 . If the GHSV is less than 2,000 h -1 , the amount of the catalyst used is increased though the denitrating ratio is high and, on the other hand, if it is greater than 200,000 h -1 , the denitrating ratio is lowered. Exhaust gases used according to the present invention are those gases containing NO x and oxygen and are exhaust gases discharged from mobile internal combustion engines such as gasoline cars which perform lean combustion condition or diesel cars, stationary internal combustion engines such as cogeneration, boilers, and various kinds of industrial furnaces. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) Example 1 A solution comprising 7.6 g of aluminum sulfate, 6.9 g of gallium nitrate, 26.4 of tetrapropylammonium bromide, 15.0 g of sulfuric acid (97%) and 250 ml of water (referred to as a solution I); a solution comprising 214 g of water glass (SiO 2 : 28.4%, Na 2 O: 9.5%) and 212 ml of water (referred to as a solution II); and a solution comprising 80 g of sodium chloride and 122 ml of water (referred to as a solution III) were at first provided. Then, the solutions I and II were gradually dropped into and mixed with the solution III. The mixed solution was adjusted to Ph 9.5 with sulfuric acid, was charged in a one liter autoclave and left for 20 hours under an autogenous pressure at a temperature of 170°C and under a stirring at 300 rpm. The mixed solution was cooled, filtered and precipitates were washed sufficiently with an excess amount of purified water. Subsequently, they were dried at 120°C for 20 hours to synthesize a galloaluminosilicate zeolite of a ZSM-5 structure. Subsequently, the zeolite was calcined in an air stream at 540°C for 3 hours. Then, it was subjected to ion-exchange by using 1N-NH 4 NO 3 solution at 80°C for 2 hours, filtration, washing with water, drying at 120°C and calcination in an air stream at 720°C for 3 hours. Subsequently, the catalyst precursor was impregnated with a K 2 CO 3 solution to support K on the precursor. The catalyst precursor thus obtained had an elemental composition of SiO 2 :Al 2 O 3 :Ga 2 O 3 :K 2 O = 95.2:2.0:2.6:0.2 by weight ratio converted as oxide and SiO 2 :Al 2 O 3 :Ga 2 O 3 :K 2 O = 79:1:0.7:0.1 by molar ratio. Then, after charging 60 cm 3 of the catalyst precursor into a tubular reactor made of stainless steel, temperature was gradually elevated while for introducing dry air at GHSV = 5,000 h -1 , and the precursor was treated at 500°C for 30 min to prepare a catalyst of this example. Then, a gas formed by adding a LPG gas to a diesel exhaust gas as a gas for disposal was introduced at GHSV = 5,000 h -1 through a tube kept at 200°C into the tubular reactor kept at 500°C and an evaluation test was conducted for the catalyst. The composition of the diesel exhaust gas was: NO x : 1000 ppm, O 2 : 8%, SO x : 140 ppm, CO: 400 ppm, CO 2 : 10% and THC: 230 ppm. Further, the LPG gas was added by 0.19% at the THC concentration such that the total hydrocarbon in the gas for disposal was 0.21% of the total THC concentration. Accordingly, the total THC concentration/NO x concentration was 2.1. Then, the gas from the exit of the tubular reactor was introduced through a tube kept at 200°C into a chemical luminescence analyzer and the NO x concentration was measured. The NO x eliminating ratio of the exhaust gas after the catalytic reaction was calculated by measuring and comparing the NO x concentration before and after the introduction to the tubular reactor. The results are shown in Table 1 below. Examples 2, 3 In Examples 2 and 3, the composition of the gas for disposal introduced into the tubular reactor charged with the catalyst upon evaluation test for the catalyst in Example 1 was changed as shown below. The results of the evaluation of the NO x eliminating ratio in each of the examples are shown in Table 1. The gas for disposal introduced in Example 2 was obtained by adding an LPG gas by 0.40% at the THC concentration to the diesel exhaust gas in Example 1 such that the total hydrocarbon was 0.42% at the total THC concentration in the gas for disposal. The gas for disposal introduced in Example 3 was obtained by adding an LPG gas by 0.82% at the THC concentration to the diesel exhaust gas in Example 1 such that the total hydrocarbon was 0.84% at the total THC concentration in the gas for disposal. Example 4 In this example, a Ba(NO 3 ) 2 solution was used instead of the K 2 CO 3 solution upon preparing the catalyst in Example 1. The catalyst precursor in this example had an elemental composition of SiO 2 :Al 2 O 3 :Ga 2 O 3 :BaO = 95.2:2.0:2.6:0.2 by weight ratio when converted to an oxide and SiO 2 :Al 2 O 3 :Ga 2 O 3 :BaO = 79:1:0.7:0.07 by molar ratio. Subsequently, the NO x eliminating ratio was evaluated in the same manner as in Example 1 and the results are shown in Table 1. Examples 5, 6 In Examples 5 and 6, the composition of the gas for disposal introduced in the tubular reactor charged with the catalyst upon evaluation test of the catalyst in Example 4 was changed as shown below. The results of the evaluation of the NO x eliminating ratio in each of the examples are shown in Table 1. The gas for disposal introduced in Example 5 was obtained by adding an LPG gas by 0.40% at the THC concentration to the diesel exhaust gas in Example 4 such that the total hydrocarbon was 0.42% at the total THC concentration in the gas for the disposal. The gas for disposal introduced in Example 6 was obtained by adding an LPG gas by 0.82% at the THC concentration to the diesel exhaust gas in Example 4 such that the total hydrocarbon was 0.84% at the total THC concentration in the gas for disposal. Example 7 In this example, the concentration of K 2 CO 3 in the K 2 CO 3 solution was so changed that the amount of K supported in the catalyst was decreased by one half upon preparing the catalyst in Example 1. The catalyst precursor in this example had an elemental composition of SiO 2 :Al 2 O 3 :Ga 2 O 3 :K 2 O = 95.3:2.0:2.6:0.1 by weight ratio when converted to an oxide and SiO 2 :Al 2 O 3 :Ga 2 O 3 :K 2 O = 80:1:0.7:0.05 by molar ratio. Subsequently, the NO x eliminating ratio was evaluated in the same manner as in Example 1 and the results are shown in Table 1. Example 8 In this example, the concentration of K 2 CO 3 in the K 2 CO 3 solution was so changed that the amount of K supported in the catalyst was increased to by a factor of 2.5 upon preparing the catalyst in Example 1. The catalyst precursor in this example had an elemental composition of SiO 2 :Al 2 O 3 :Ga 2 O 3 :K 2 O = 94.9:2.0:2.6:0.5 by weight ratio when converted to an oxide and SiO 2 :Al 2 O 3 :Ga 2 O 3 :K 2 O = 79.1:1:0.7:0.25 by molar ratio. Subsequently, the NO x eliminating ratio was evaluated in the same manner as in Example 1 and the results are shown in Table 1. Comparative Example Using the catalyst as prepared in Example 1, the same evaluation test of the catalyst as in Example 1 was conducted by introducing a diesel exhaust gas to which the LPG gas was not added into the tubular reactor. The results are shown in Table 2. Consideration for Examples and Comparative Example As shown in Examples 1 to 8, according to the method of purifying the exhaust gas of the examples, it can be seen that as high a denitrating ratio as 50 to 82% was obtained even when the oxygen concentration in the exhaust gases is as high as 8% since the catalyst in which K or Ba as the promoter was supported on the Ga-containing support galloaluminosilicate as the main catalyst was used and the evaluation test was conducted by setting the temperature at 500°C and the total THC concentration/NO x concentration at 2.1 to 8.4 during the catalytic reaction. In contrast, according to the Comparative Example, though the same catalyst as that used in Examples 1 to 3 was used, only as low a denitrating ratio as 4% was obtained since the LPG was not added and the THC concentration/NO x concentration was 0.2 which was lower than the range according to the present invention in the evaluation test. The catalyst used according to the present invention has high catalytic activity even at a low temperature and has a long catalyst life. Further, using the above-mentioned catalyst, nitrogen oxides can be reduced and eliminated at high efficiency even if the concentration of oxygen in the exhaust gas is high.

Claims: Use of a catalyst comprising (1) gallium, (2) at least one of alkali metal elements and alkaline earth metal elements and (3) a zeolite, for the elimination of nitrogen oxides from an exhaust gas under the presence of a hydrocarbon in an oxidative atmosphere. The use of claim 1, wherein said (1) gallium and said (3) zeolite are present in the form of galloaluminosilicate zeolite.

Inventors: HAYASAKA TOSHIAKI, KIMURA TAKUMA, HAYASAKA, TOSHIAKI, KIMURA, TAKUMA, Hayasaka, Toshiaki, c/o IDEMITSU KOSAN CO. LTD., Kimura, Takuma, c/o IDEMITSU KOSAN CO. LTD.
Applicants: IDEMITSU KOSAN CO, IDEMITSU KOSAN COMPANY LIMITED
Application year: 1991
Main classification: B01D53
IPC codes: B01D53, B01J29